APT is a package management system for Debian and other Linux distributions based on it, such as Ubuntu. For the most part, APT is easy to use for installing, removing, and updating packages. In rare instances, often when you are mixing in third-party dependencies, there is a chance that apt-get may end up giving you an error telling you that a package installation could not be completed.

Solutions:

It is always a good idea to back up configuration files like /etc/apt/sources.list, so you can revert the changes if needed.

• If the error shows something like this:

  <some-package>: Depends: <other-package> (= version) but this-version is to be installed

  Then make sure that the restricted and universe repositories are enabled. Hit Alt+F2, type software-properties-gtk and hit Enter.

  Under Ubuntu Software tab, enable all the repositories.

  

• One possible cause of unmet dependencies could be corrupted package database, and/or some packages weren’t installed properly. To fix this problem, hit Alt+Ctrl+T to open terminal and try to run one of the following commands:

  sudo apt-get clean

  or,

  sudo apt-get autoclean

  apt-get clean clears out the local repository of retrieved package files (the .deb files). It removes everything but the lock file from /var/cache/apt/archives/ and /var/cache/apt/archives/partial/. apt-get autoclean clears out the local repository of retrieved package files, but unlike apt-get clean, it only removes package files that can no longer be downloaded, and are largely useless.

• One of the most basic fixes to resolve dependencies problems is to run:

  sudo apt-get -f install

  The -f here stands for “fix broken”. Apt will attempt to correct broken dependencies. If you manually installed a package that had unmet dependencies, apt-get will install those dependencies, if possible, otherwise it may simply remove the package that you installed in order to resolve the problem.

  Then run:

  sudo dpkg --configure -a

  Then run this again:

  sudo apt-get -f install

  If the output is:

  0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

  That means it failed.

• Next solution is to run:

  sudo apt-get -u dist-upgrade

  If it shows any held packages, it is best to eliminate them. Packages are held because of dependency conflicts that apt cannot resolve. Try this command to find and repair the conflicts:

  sudo apt-get -o Debug::pkgProblemResolver=yes dist-upgrade

  If it cannot fix the conflicts, it will exit with:

  0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.

  Delete the held packages one by one, running dist-upgrade each time, until there are no more held packages. Then reinstall any needed packages. Be sure to use the --dry-run option, so that you are fully informed of consequences:

  sudo apt-get remove --dry-run package-name

  Since removing the package you are trying to install may not be ideal, you might also try finding a repository that has the packages you need to satisfy the dependencies.

Finally, if all else fails, you can attempt to satisfy the dependencies yourself, either by finding and installing the necessary packages, or by installing them from source and then creating “deb” packages for them.

Disable/Remove/Purge PPAs:

PPAs (Personal Package Archive) are repositories hosted on Launchpad. You can use PPAs to install or upgrade packages that are not available in the official Ubuntu repositories.

One of the most common causes of unmet dependencies are PPAs, especially when used to upgrade the existing package in Ubuntu repositories. To solve the problem you have three options: disable, purge (revert back to original package in Ubuntu repositories) or remove PPA.

• Disable:

  Disabling a PPA means no more updates for the packages installed from that PPA. To disable a PPA: Open Software Center > Edit > Software Sources Or,

  Hit Alt+F2 and run software-properties-gtk.

  Click on Other Software tab, you'll see that each PPA have two lines here, one for the compiled packages and one for the source, Uncheck both lines to disable a PPA. Here you can also add and remove PPAs.

  

• Purge:

  Purging a PPA means, downgrading the packages in the selected PPA to the version in the official Ubuntu repositories and disabling that PPA. PPA Purge does exactly that. To install PPA Purge run the following command:

  sudo apt-get install ppa-purge

But, considering the question apt is broken, the above command will fail. So use this command

mkdir ppa-purge && cd ppa-purge && wget http://mirror.pnl.gov/ubuntu/pool/universe/p/ppa-purge/ppa-purge_0.2.8+bzr56_all.deb && wget http://mirror.pnl.gov/ubuntu//pool/main/a/aptitude/aptitude_0.6.6-1ubuntu1_i386.deb && sudo dpkg -i ./*.deb

To use PPA Purge:

sudo ppa-purge ppa:someppa/ppa

If 'ppa-purge' command fails for some reason, you can't run 'ppa-purge' again unless you re-enable the PPA (To enable the PPA follow the same steps as disabling a PPA and Check the both lines of a particular PPA to enable it).

Note: PPA Purge doesn't remove PPAs (may be in future), So you'll have to manually remove the PPA.

• Remove:

  If the package installed from a PPA doesn't exist in official Ubuntu repositories then using PPA Purge is not recommended, because there's nothing to be downgraded and PPA Purge wouldn't delete it either. To remove a PPA and installed packages run the following commands: (Ignore the first command if you don't want to remove the installed packages)

  sudo apt-get autoremove --purge package-name
  sudo add-apt-repository --remove ppa:someppa/ppa
  sudo apt-get autoclean
  

Alternatively you can use Y PPA Manager to disable/remove/purge PPA's (Details at the end of answer).

Preventive Measures:

So how can we avoid this from happening in the first place?

• Keep Ubuntu Up to date. Ubuntu automatically notifies when updates are available, you can also check for available updates by clicking on Session Indicator in Unity panel:

  

  Or, Hit Alt+Ctrl+T to open terminal and run following commands:

  sudo apt-get update
  sudo apt-get upgrade

  Update: Synchronizes your list of available packages with the servers in source repositories. Upgrade: Downloads & installs any newer versions of your installed packages.

• If you decide to add other repositories to sources.list, make sure that the repository is meant to work (and known to work) with Ubuntu. Repositories that are not designed to work with your version of Ubuntu can introduce inconsistencies in your system and might force you to re-install. Also, make sure that you really need to add external repositories as the software package(s) you are looking for may already have been introduced into the official repositories! Source

• Remove duplicate PPAs. Y PPA Manager (installation guide at the end of answer) can easily scan and remove duplicate PPAs.

  • Hit Alt+F2 and run y-ppa-manager to open Y PPA Manager.
  • Once open, double click or hit Enter on Advanced.
  • In resulting window select Scan and remove duplicate PPAs and click OK.

Getting Help:

When asking for help on Askubuntu or any other help forum, you should include the output of the following commands in your question:

• Output of actual error:

  sudo apt-get install package-name

• This will show your sources.list:

  cat /etc/apt/sources.list

• This will show the list of PPAs (If any):

  cat /etc/apt/sources.list.d/*

Additional Sources:

• 'apt' Ubuntu Manpage
• 'apt-get' Ubuntu Manpage
• 'apt-get/how-to' Community Ubuntu documentation
• 'Repositories/Ubuntu' Community Ubuntu documentation
• 'Repositories/CommandLine' Community Ubuntu documentation

---

Y PPA Manager: Y PPA Manager is an one stop shop for all of your PPA needs.

Here are some of its features:

• search packages in all Launchpad PPAs
• list and download packages in a PPA
• add / remove / purge a PPA
• backup and restore PPA sources
• remove duplicate PPA sources

To install Y PPA Manager, open terminal by hitting Alt+Ctrl+T and run following commands:

sudo add-apt-repository ppa:webupd8team/y-ppa-manager
sudo apt-get update
sudo apt-get install y-ppa-manager

Considering the question, apt is broken, so use these commands instead

sudo su

and

32 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_i386.deb && dpkg -i ./*.deb

64 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_amd64.deb && dpkg -i ./*.deb

Note: All commands asked to be run must be run in the terminal, which can be opened by either Ctrl+Alt+T or searching for terminal in the dash.

Is it really broken?

Try running the following command and try to reinstall the software you were trying to install

sudo apt-get update

---

Pre-Perfomance Steps

Backing up

Back up the following files:

• /etc/apt/sources.list
• /var/lib/dpkg/status

To do so, use these commands

sudo cp /etc/apt/sources.list /etc/apt/sources.list.original

and

sudo cp /var/lib/dpkg/status /var/lib/dpkg/status.original

Clearing your apt-cache

apt keeps a cache of recently downloaded packages to save bandwidth when it is required to be installed. This can be counter-productive in some cases

Now, to clean it, you have two options

sudo apt-get clean 

This will remove all cached packages belonging to the folder /var/cache/apt/archives/ and /var/cache/apt/archives/partial except the .lock files. This is recommended

sudo apt-get autoclean

This scans the folders /var/cache/apt/archives/ and /var/cache/apt/archives/partial and checks if the package is still in the repositories and removes the ones that aren't

---

Fixing dependencies

Using apt's fix-broken mode

sudo apt-get -f install

This will cause apt to scan for missing dependencies and fix them from the repositories

If the output states that nothing new was installed or upgraded, it has failed.

---

Checking if all required sources are enabled

Type gksu software-properties-gtk and you'll get this window

Make sure all sources are enabled.

next, go to the Other software tab and check if the required PPAs for the software to be installed are there and are enabled. Also, try disabling some PPAs which might be having broken packages

now, run sudo apt-get update

Try installing the software now

Selecting a better server to download from

Type gksu software-properties-gtk and you'll get this window

Click the Download from the Dropdown box and select other

Click Select Best Server

Run sudo apt-get update

Try installing the software

also, try using sudo apt-get install -f

---

PPA Purge

This is a tool used to purge broken/unwanted ppa's and their applications along with it

To install it, run

sudo apt-get install ppa-purge

But, Considering the question apt is broken so the above command will fail. So use this command

mkdir ppa-purge && cd ppa-purge && wget http://mirror.pnl.gov/ubuntu/pool/universe/p/ppa-purge/ppa-purge_0.2.8+bzr56_all.deb && wget http://mirror.pnl.gov/ubuntu//pool/main/a/aptitude/aptitude_0.6.6-1ubuntu1_i386.deb && sudo dpkg -i ./*.deb

Now use ppa purge

sudo ppa-purge ppa:someppa/ppa  

---

Y-PPA Manager

Y-PPA Manager is a gui app that helps you manage PPA's and various problems assosiated with it

To install it

sudo add-apt-repository ppa:webupd8team/y-ppa-manager

and

sudo apt-get update

and

sudo apt-get install y-ppa-manager

Considering the question, apt is broken so, use these command instead

sudo su

and

32 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_i386.deb && dpkg -i ./*.deb

64 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_amd64.deb && dpkg -i ./*.deb

Now type in y-ppa-manager

You'll be presented with this window

Double click on advanced, and you'll get this window

Do the following Tasks outlined in black

---

Prevention is better than cure

It is better to prevent than to search for this question on AskUbuntu :D

So, here are the guidelines to keep you safe

Keep your system up-to-date

always run the following command regularly

sudo apt-get update&&sudo apt-get upgrade

or, you can always use Update Manager with this command

gksu update-manager

Using only trusted PPA's

Only use PPA's meant to be used on Ubuntu also, only use PPA's with trusted sources. Infact, the package might already be in the ubuntu repositories

Backing up when things are good and restoring it later

For this you need Y-PPA-Manager. The steps to install it are given above.

Run this command to open Y-PPA-Manager

y-ppa-manager

You'll be presented with this window

Double click on advanced, and you'll get this window

Run this:

You'll be asked to save a tar.gz file with a dialog similar to the one below. Save it in another partition or a safe place

Later, when you need to restore it again, follow similar steps and when you get to the advanced dialog,Click on this:

You'll be asked to restore from the previous backup which you saved before with a dialog similar to the one below

---

Still not working?

Package dependency errors are not always generic and depends on the package to be installed.

If following all the steps given to fix the error does not work for you, you can always ask on Ask Ubuntu

Here are some commands which you need to post the output of

sudo apt-get install packagename

and

cat /etc/apt/sources.list

and

cat /etc/apt/sources.list.d/*

(Thanks to Basharat Sial)

There are also other files/commands that you need the output of that might be error specific, and users will probably prompt you in the comments to post the file/command.

These answers so far are focused on how to help the system automagically resolve such an issue, mostly hoping that it resolves itself, but not focused on understanding how to investigate dependencies. It's important to try making sure your package lists are up to date first; always make sure a simple update or clearing of the cache doesn't resolve the issue before digging deeper. If everything behind the scenes is working as it should, however, it's paramount to understand dependencies. Sometimes the layers of dependencies can seem overwhelming, but the concept is simple and key to understanding how software is installed/uninstalled in an Ubuntu system.

I highly recommend aptitude to anyone using a .deb based system, such as Debian, Kali, Ubuntu or any of its derivatives. It provides an interface to browse through layers of dependencies, including recommended and suggested packages. It is an invaluable tool for when apt-get doesn't offer a solution to a conflict and for visualizing dependencies in general. (Synaptic is also available in Ubuntu.)

Aptitude's conflict resolver will walk you through the possible combinations of installed/removed/upgraded/etc packages that satisfy your requested actions without conflict. This can often be successful even when apt is unable to find a solution. As an Ubuntu user, situations don't normally arise that require you to investigate why a particular package is being installed or causing a conflict, but it's almost inevitable as you use the system more.

package1 : Depends: package2 (>= 1.8) but 1.7.5-1ubuntu1 is to be installed

Explanation of the message:

1. In order for your installation to be completed, package1 needs package2.
2. package2 has to be available in version >= 1.8
3. The package manager checked all repositories and only found version 1.7.5-1ubuntu1 that could be installed, which it won't do because it is < 1.8

Fix

There is NO single right answer for this question and there is NO simple answer.

There are many scenarios why this can happen and one has to investigate all options and try to solve the preconditions that lead to this error.

Scenario A:

All the repositories that you have access to don't have package2 in version >= 1.8, so you will have to find a repository that does. If you are not able to find a repository with versions >=1.8 then you might have to build and install package2 from source. And then try to install package1.

If you want to know how to build a package from source, check other questions/answers or post a new one.

Scenario B:

The repositories do include package2 in a version >= 1.8 but the package manager still wants to install an older version (1.7.5).

The reason for this may be that the later versions of package2 where built by some newer technology that is not supported by other packages that you already have installed.

This might not be possible to fix at all. You can try to use apt or synaptic and look for all versions of the package2 that are available in the repositories, and try to install a specific version >= 1.8

If you have other incompatible packages and you do this, you will get a list of similar error messages for all the other packages that prevent the installation of package2>=1.8.

In this case, you have a bigger problem. You wanted to install package1, which needs package2 to be >= 1.8, but package2 in version >=1.8 requires package3 to be in version >= 6.7 and you have package3 version 6.3

The thing is that the package manager (APT) tries to solve all these dependencies automatically if it is possible. When it says that it is not possible (and issues a message like in the question), then you should trust it.

If you decide not to trust the package manager and if you are sure that you will not have problems, then you can try to force install the specific needed versions of all packages, one by one, and hope to be OK.

Beware, doing this for system packages can create serious problems. You can lock yourself out of the system, block critical services etc.

Scenario C:

No matter the reasons for the problem, you can always setup for manual compilation and build the needed package and all of it's dependencies in the exact needed versions from source. With this option you can keep what you built in a separate directory and out of the system path so that it will not create conflicts with the system. This is probably the cleanest solution, and sometimes can be faster to do.

Scenario D:

Package manager has access to out-dated information about packages and therefor cannot find a solution. Look in the other answers.

Scenario E:

Package manager has a bug in the dependency resolution code.

You have to investigate that this is really the case, and then issue a bug request.

...

Scenario Z:

There might be other scenarios.

For example package maintainers could have been wrong and did not check that package1 would work even with version of package2 < 1.8 and in the specification they set wrong dependencies.

These methods need to be followed whenever you get eror regarding Unmet Dependencies

After following the instructions try installing the software. Atleast one of these methods should work for you

1. Before trying any new command or adding any new ppa try this

Cleaning up of the apt cache:

sudo apt-get clean

Cleaning up of any unused dependencies:

sudo apt-get autoremove

Updating apt-cache

sudo apt-get update

This will remove all the cache and will download a fresh cache. This should be done to get fresh cache packages and newer updates.

2. Try sudo apt-get install -f.

It will probably give some suggestions about removing and installing packages, do what seems reasonable.

3. Change the Download Server from Software Sources Because sometimes the files on a server are corrupted.

4. This also happens if you before added an external ppa for the broken dependency.Try removing that ppa by

• either typing sudo add-apt-repository --remove ppa:someppa/ppa and then manually removing the ppa installed files/libraries

or

• purging a ppa (this removes the installed files also)

  sudo apt-get install ppa-purge   #a one time install 
  sudo ppa-purge ppa:someppa/ppa  
  

5. If no 4 is not your case then atleast try to remove the package that is being shown as a broken dependency.Then try again installing.This is reported to have workeed for quite a few people.

6. Always add the stable version of a repository/PPA.

7 Also make sure you have all the official repositories enabled (In Software Sources)

Please do not enable the Proposed Updates as they are meant for testing/developers.

---

As told by doug:

Usually the best 1st step to resolving this is to identify the source of <pkg1> & <pkg2>.

Additionally not noted is that in many, if not most cases, "<whatever> is to be installed" really means "<whatever> is installed" and should be tried to be removed if nothing else works (as in point 5).

---

I'll keep adding more things ill find to fix unmet dependencies.

For most cases,installing the package through aptitude will resolve unmet dependencies.By default aptitude package was not installed on your pc.So first install it by running,

sudo apt-get update
sudo apt-get install aptitude

then,

sudo aptitude install packagename

For Example,

    
avinash@avinash-Lenovo-IdeaPad-Z500:~$ sudo apt-get install ubuntu-sdk
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ubuntu-sdk : Depends: qtcreator-plugin-ubuntu but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Installing the package through aptitude

avinash@avinash-Lenovo-IdeaPad-Z500:~$ sudo aptitude install ubuntu-sdk
The following NEW packages will be installed:
  apparmor-easyprof{a} apparmor-easyprof-ubuntu{a} at{a} build-essential{a} 
  click{a} click-apparmor{a} click-dev{a} click-doc{a} cordova-ubuntu-2.8 
  cordova-ubuntu-2.8-examples{a} debhelper{a} devscripts{a} dh-apparmor{a} 
  dh-make{a} distro-info-data{a} dpkg-dev{a} dput{a} g++{a} g++-4.8{a} 
  geoclue-gypsy{a} libalgorithm-diff-perl{a} libalgorithm-diff-xs-perl{a} 
  libalgorithm-merge-perl{a} libboost-filesystem1.53.0{a} 
  libboost-program-options1.53.0{a} libboost-regex1.53.0{a} 
  libbotan-1.10-0{a} libdee-qt5-3{a} libdistro-info-perl{a} 
  libexporter-lite-perl{a} libglib2.0-dev{a} 
  libgstreamer-plugins-base1.0-dev{a} libgstreamer1.0-dev{a} libgypsy0{a} 
  libhud2{a} libio-stringy-perl{a} libjs-jquery{a} libjs-sphinxdoc{a} 
  libjs-underscore{a} liblockdev1{a} libmail-sendmail-perl{a} 
  libparse-debcontrol-perl{a} libpcre3-dev{a} libpcrecpp0{a} 
  libpoppler-qt5-1{a} libqdjango-db0{a} libqgsttools-p1{a} 
  libqt5clucene5{a} libqt5contacts5{a} libqt5declarative5{a} 
  libqt5designer5{a} libqt5designercomponents5{a} libqt5feedback5{a} 
  libqt5help5{a} libqt5location5-plugins{a} libqt5multimedia5{a} 
  libqt5multimedia5-plugins libqt5multimediaquick-p5{a} 
  libqt5multimediawidgets5{a} libqt5organizer5{a} 
  libqt5qml-graphicaleffects{a} libqt5script5 libqt5scripttools5{a} 
  libqt5svg5{a} libqt5svg5-dev libqt5systeminfo5{a} libqt5v8-5-dev 
  libqt5versit5{a} libqt5versitorganizer5{a} libqt5webkit5-dev 
  libqt5webkit5-qmlwebkitplugin{a} libqt5xmlpatterns5{a} 
  libqt5xmlpatterns5-dev libstdc++-4.8-dev{a} libsys-hostname-long-perl{a} 
  libthumbnailer0{a} libu1db-qt5-3{a} libunity-action-qt1{a} 
  libupstart-app-launch1{a} libusermetricsinput1{a} libxml2-dev{a} 
  libxslt1-dev{a} po-debconf{a} python3-apparmor-click{a} python3-click{a} 
  python3-magic{a} qmlscene qt3d5-dev qtbase5-doc{a} qtcreator{a} 
  qtcreator-doc{a} qtcreator-plugin-ubuntu{a} 
  qtcreator-plugin-ubuntu-common{a} qtdeclarative5-accounts-plugin 
  qtdeclarative5-cordova-2.8-plugin{a} qtdeclarative5-dev-tools 
  qtdeclarative5-doc-html{a} qtdeclarative5-folderlistmodel-plugin 
  qtdeclarative5-friends0.2 qtdeclarative5-localstorage-plugin 
  qtdeclarative5-particles-plugin qtdeclarative5-poppler1.0 
  qtdeclarative5-qtaudioengine-plugin qtdeclarative5-qtfeedback-plugin{a} 
  qtdeclarative5-qtlocation-plugin qtdeclarative5-qtmultimedia-plugin{a} 
  qtdeclarative5-qtorganizer-plugin qtdeclarative5-qtquick2-plugin{a} 
  qtdeclarative5-qtsensors-plugin qtdeclarative5-u1db1.0 
  qtdeclarative5-ubuntu-ui-extras-browser-plugin{a} 
  qtdeclarative5-ubuntu-ui-extras-browser-plugin-assets{a} 
  qtdeclarative5-ubuntu-ui-toolkit-plugin{a} 
  qtdeclarative5-unity-action-plugin{a} qtdeclarative5-usermetrics0.1 
  qtdeclarative5-window-plugin{a} qtdeclarative5-xmllistmodel-plugin 
  qtgraphicaleffects5-doc{a} qtlocation5-dev{a} qtmultimedia5-dev 
  qtmultimedia5-doc{a} qtmultimedia5-doc-html{a} qtscript5-dev 
  qtscript5-doc{a} qtscript5-doc-html{a} qtsensors5-dev{a} qtsvg5-doc{a} 
  qtsvg5-doc-html{a} qttools5-dev qttools5-dev-tools qttools5-doc{a} 
  qtwebkit5-doc-html{a} schroot{a} schroot-common{a} sqlite3{a} 
  ubuntu-html5-container ubuntu-html5-sdk{ab} ubuntu-html5-theme{a} 
  ubuntu-html5-theme-examples ubuntu-sdk ubuntu-sdk-libs 
  ubuntu-sdk-libs-dev{a} ubuntu-ui-toolkit-doc{a} 
  ubuntu-ui-toolkit-examples{a} ubuntu-ui-toolkit-theme{a} 
  unity-action-doc{a} upstart-app-launch{a} upstart-app-launch-tools{a} 
  usermetricsservice{a} wdiff{a} zlib1g-dev{a} 
The following packages will be upgraded:
  cpp-4.8 gcc-4.8 gcc-4.8-base hud libasan0 libatomic1 libgcc-4.8-dev 
  libgcc1 libglib2.0-0 libglib2.0-bin libgomp1 libitm1 libquadmath0 
  libstdc++6 libtsan0 
15 packages upgraded, 151 newly installed, 0 to remove and 293 not upgraded.
Need to get 108 MB/126 MB of archives. After unpacking 273 MB will be used.
The following packages have unmet dependencies:
 ubuntu-html5-sdk : Depends: qtdeclarative5-ubuntu-webapps-api which is a virtual package.
                    Depends: ubuntu-html5-ui-toolkit but it is not going to be installed.
The following actions will resolve these dependencies:

     Keep the following packages at their current version:              
1)     qtcreator-plugin-ubuntu [Not Installed]                          
2)     ubuntu-html5-sdk [Not Installed]                                 
3)     ubuntu-sdk [Not Installed]                                       

     Leave the following dependencies unresolved:                       
4)     libhud-client2 recommends hud (= 13.10.1+13.10.20131014-0ubuntu1)
5)     qtcreator-plugin-ubuntu-common recommends qtcreator-plugin-ubuntu


Accept this solution? [Y/n/q/?] y
The following NEW packages will be installed:
  apparmor-easyprof{a} apparmor-easyprof-ubuntu{a} build-essential{a} 
  click{a} click-apparmor{a} cordova-ubuntu-2.8 debhelper{a} dh-apparmor{a} 
  dh-make{a} dpkg-dev{a} g++{a} g++-4.8{a} geoclue-gypsy{a} 
  libalgorithm-diff-perl{a} libalgorithm-diff-xs-perl{a} 
  libalgorithm-merge-perl{a} libdee-qt5-3{a} libglib2.0-dev{a} 
  libgstreamer-plugins-base1.0-dev{a} libgstreamer1.0-dev{a} libgypsy0{a} 
  libhud2{a} libjs-jquery{a} libmail-sendmail-perl{a} libpcre3-dev{a} 
  libpcrecpp0{a} libpoppler-qt5-1{a} libqdjango-db0{a} libqgsttools-p1{a} 
  libqt5clucene5{a} libqt5contacts5{a} libqt5designer5{a} 
  libqt5designercomponents5{a} libqt5feedback5{a} libqt5help5{a} 
  libqt5location5-plugins{a} libqt5multimedia5{a} libqt5multimedia5-plugins 
  libqt5multimediaquick-p5{a} libqt5multimediawidgets5{a} 
  libqt5organizer5{a} libqt5qml-graphicaleffects{a} libqt5script5 
  libqt5scripttools5{a} libqt5svg5{a} libqt5svg5-dev libqt5systeminfo5{a} 
  libqt5v8-5-dev libqt5versit5{a} libqt5versitorganizer5{a} 
  libqt5webkit5-dev libqt5webkit5-qmlwebkitplugin{a} libqt5xmlpatterns5{a} 
  libqt5xmlpatterns5-dev libstdc++-4.8-dev{a} libsys-hostname-long-perl{a} 
  libthumbnailer0{a} libu1db-qt5-3{a} libunity-action-qt1{a} 
  libupstart-app-launch1{a} libusermetricsinput1{a} libxml2-dev{a} 
  libxslt1-dev{a} po-debconf{a} python3-apparmor-click{a} python3-click{a} 
  qmlscene qt3d5-dev qtdeclarative5-accounts-plugin 
  qtdeclarative5-cordova-2.8-plugin{a} qtdeclarative5-dev-tools 
  qtdeclarative5-folderlistmodel-plugin qtdeclarative5-friends0.2 
  qtdeclarative5-localstorage-plugin qtdeclarative5-particles-plugin 
  qtdeclarative5-poppler1.0 qtdeclarative5-qtaudioengine-plugin 
  qtdeclarative5-qtfeedback-plugin{a} qtdeclarative5-qtlocation-plugin 
  qtdeclarative5-qtmultimedia-plugin{a} qtdeclarative5-qtorganizer-plugin 
  qtdeclarative5-qtquick2-plugin{a} qtdeclarative5-qtsensors-plugin 
  qtdeclarative5-u1db1.0 qtdeclarative5-ubuntu-ui-extras-browser-plugin{a} 
  qtdeclarative5-ubuntu-ui-extras-browser-plugin-assets{a} 
  qtdeclarative5-ubuntu-ui-toolkit-plugin{a} 
  qtdeclarative5-unity-action-plugin{a} qtdeclarative5-usermetrics0.1 
  qtdeclarative5-window-plugin{a} qtdeclarative5-xmllistmodel-plugin 
  qtlocation5-dev{a} qtmultimedia5-dev qtscript5-dev qtsensors5-dev{a} 
  qttools5-dev qttools5-dev-tools sqlite3{a} ubuntu-html5-container 
  ubuntu-html5-theme{a} ubuntu-html5-theme-examples ubuntu-sdk-libs 
  ubuntu-ui-toolkit-theme{a} upstart-app-launch{a} 
  upstart-app-launch-tools{a} usermetricsservice{a} wdiff{a} zlib1g-dev{a} 
The following packages will be upgraded:
  cpp-4.8 gcc-4.8 gcc-4.8-base hud libasan0 libatomic1 libgcc-4.8-dev 
  libgcc1 libglib2.0-0 libglib2.0-bin libgomp1 libitm1 libquadmath0 
  libstdc++6 libtsan0 
15 packages upgraded, 108 newly installed, 0 to remove and 293 not upgraded.
Need to get 27.1 MB/45.8 MB of archives. After unpacking 104 MB will be used.
Do you want to continue? [Y/n/?] 

My answer is similar to rick's above.

Using apt-get clean, apt-get autoclean, apt-get autoremove, apt-get upgrade, apt-get update, apt-get -u dist-upgrade, etc. all didn't work for me.

I finally resolved this issue by descending into the dependency tree via apt-get install, illustrated with an example below. In my case, I wanted to install libmagickwand-dev:

apt-get install libmagickwand-dev

I got this error:

The following packages have unmet dependencies:
 libmagickwand-dev : Depends: libmagickcore4-extra (= 8:6.6.9.7-5ubuntu3.2) but it is not going to be installed
                     Depends: libmagickcore-dev (= 8:6.6.9.7-5ubuntu3.2) but it is not going to be installed

So, I ran apt-get install for one of the failing dependencies:

apt-get install libmagickcore-dev

The following packages have unmet dependencies:
 libmagickcore-dev : Depends: libmagickcore4-extra (= 8:6.6.9.7-5ubuntu3.2) but it is not going to be installed
                     Depends: libgraphviz-dev but it is not going to be installed

I descended further down the tree:

apt-get install libgraphviz-dev

with another error:

The following packages have unmet dependencies:
 libgraphviz-dev : Depends: libgvc5 (= 2.26.3-10ubuntu1) but it is not going to be installed

Once more deeper down the tree:

apt-get install libgvc5

Fore some strange reason, this succeeded, and I was able to install the original package I wanted with apt-get install libmagickwand-dev:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  libcdt4 libgd2-xpm libgraph4 libpathplan4
Suggested packages:
  libgd-tools
The following NEW packages will be installed:
  libcdt4 libgd2-xpm libgraph4 libgvc5 libpathplan4
0 upgraded, 5 newly installed, 0 to remove and 2 not upgraded.
Need to get 808 kB of archives.
After this operation, 2,320 kB of additional disk space will be used.
Do you want to continue [Y/n]?

Excellent answers! But today my case seems different from yours. Here's my advice.

Problem:

I failed to install apache2 in ubuntu 10.10.

rick@wen-VirtualBox:~$ sudo apt-get install apache2

Reading package lists... Done

Building dependency tree      

Reading state information... Done

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

The following information may help to resolve the situation:

The following packages have unmet dependencies:

 apache2 : Depends: apache2-mpm-worker (= 2.2.16-1ubuntu3) but it is not going to be installed or

                    apache2-mpm-prefork (= 2.2.16-1ubuntu3) but it is not going to be installed or

                    apache2-mpm-event (= 2.2.16-1ubuntu3) but it is not going to be installed or

                    apache2-mpm-itk (= 2.2.16-1ubuntu3) but it is not going to be installed

           Depends: apache2.2-common (= 2.2.16-1ubuntu3) but it is not going to be installed

E: Broken packages

Solution:

My way is add the depended package into install list.

If 'sudo apt-get install apache2' failed, try 'sudo apt-get install apache2 apache2.2-common'.

See? apache2.2-common comes from error log of previous step.

At last I found there's a dependency chain like this:

 apache2.2-common <-- apache2.2-bin <-- libaprutil1-ldap <-- libaprutil1

For the first 3 packages, apt-get log told me ' is not going to be installed',

but for the last package, it shows 'libaprutil1-ldap : Depends: libaprutil1 (= 1.3.9+dfsg-3build1) but 1.3.9+dfsg-3ubuntu0.10.10.1 is to be installed'

I have a different version existing in my lib, so:

sudo apt-get remove libaprutil1

sudo apt-get install apache2

Now it's ok.

what is unmet dependencies Error

On Ubuntu packages are the primary way that software is built, deployed, and installed. Very simply, packages are an alternative to downloading, building, and installing software from scratch. They offer a host of advantages in terms of installation, removal, monitoring, and handling interactions between pieces of software over the standard "build from source" model. Ubuntu uses centralized packaging system ,Ie Every package is depends to some other packages .When we installs a package it's dependencies will be installed ,The unmet dependencies error occurs when the the dependencies of the package is not available for installation

Possible causes

1.Computer is not connected to the Internet
2.The repository containing the depandancy package is not enabled
3.Package catalog is not updated
4.The version of the depandancy in the current repository is old
5.Software index is broken
6.The depandancy is not available for ubuntu

Let's TRy to solve it

1. Connect to the internet :D

2. Make sure that all the default repositories are enabled

   ->You can easily add them in the Software Center->Edit->Software Sources->Other Sources->Add"

   -> Start the Software Center.

   ->. In the panel go to Edit and select Software Sources.

   

   ->Make sure all the repositories are checked

3. Update the software catalog

   click on the top right icon and chose update-available Entry in the menu
   

and press check button

4.type ctrl+alt+t to open terminal and type sudo apt-get install -f

5.Purge the recently added ppa using ppa-purge ppa-purge

This program disables a PPA from your Software Sources and reverts your system back to the official Ubuntu packages. You can use this to return your system to normal after testing a new version from a PPA.

Syntax

sudo ppa-purge ppa:<repository-name>/<subdirectory>

Example

sudo ppa-purge ppa:ubuntu-mozilla-daily/ppa

5. open software center ,and try to install any software , If the following dialog box appeared ,press on the repair button ,and wait

Else type ctrl+alt+t to open terminal and type sudo apt-get install -f and follow the onscreen instructions

If still error occurs, Feel free to to ask it in askbuntu

...dont forget about `build-dep`

In addition to those answers above -- I have to admit I am surprised that nobody have mentioned this approach which, is what I have always used as a "silver bullet" at any time I encountered apt-get conflicts or unresolved packages (or even if the system is being stubborn....ha!).

EDITED: using sudo dpkg -r [package] is encouraged as it has pretty powerful removal function. Since apt-get works alongside with dpkg, which means apt-get will obey dpkg function.

  sudo dpkg -r [package]
  sudo apt-get build-dep [package]  
  ## pre-install all deps for the package

You call apt-get to purge the failed package you were trying to install in the system, and then you call build-dep command, which apt-get will grab and install dependencies individually belonging to this package, so that way they'll satisfy the package prior. Then finally, call the apt-get install:

sudo apt-get install [package]

---

....'Ugh! Let's twist your arm until you yell "Uncle!"'

This rarely happened during my long years of linux use, but I found myself using this approach seldomly and succeeded. This is an example for the recent struggle I had earlier a month ago with packages mysql-client/mysql-server. After a dozen failed attempts to reset my root password with mysql-server (even via --skip-grant-tables techqiue but yet failed), I finally succeed in 'twisting Linux's arm until it yelled "Uncle!"':

sudo dpkg -r mysql-server mysql-client
sudo apt-get autoclean  
## this dumps any copy of existing package to prevent apt-get from "re-using" it.
sudo apt-get build-dep mysql-client   
 ## one package at a time, starting with client first
sudo apt-get install mysql-client
sudo apt-get build-dep mysql-server
sudo apt-get install mysql-server

apt-get autoclean is the key in this solution