This does not make your brightness function keys work, but is a workaround.

Install Brightness Controller with the following commands:

sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt update

For Version 2 with Multi Monitor Support and Color Temperature support:

sudo apt install brightness-controller

For Version 1 with up to 4 Monitor Support:

sudo apt install brightness-controller-simple

(Note: Version 1 is not anymore updated)

Note: This does not decrease the intensity of backlight, so won't save your batteries. But you can use this as a last resort to save your eyes. This also works in desktops, where there is no option to control brightness.

Hardware brightness buttons ? ?

Since Ubuntu LTS 18.04

Here is every step required for xbacklight control:

1. $ sudo nano /etc/default/grub and replace the corresponding line with GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
2. $ sudo update-grub
3. No joking, make sure that the appropriate drivers are actually installed: $ sudo apt install xbacklight xorg xserver-xorg-video-intel
4. Issuing $ find /sys -type f -name brightness should yield something like /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/brightness
5. $ cd /sys/class This directory should contain a soft link called brightness to the brightness device discovered in the previous step. Should it be missing, create it: $ sudo ln -s /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/brightness /sys/class/brightness

6. $ cat /etc/X11/xorg.conf should read:

   Section "Device"
       Identifier      "Device0"
       Driver          "intel"
       Option          "Backlight"      "intel_backlight"
   EndSection
   
   Section "Monitor"
       Identifier      "Monitor0"
   EndSection
   
   Section "Screen"
       Identifier      "Screen0"
       Monitor         "Monitor0"
       Device          "Device0"
   EndSection
   

   If it doesn't, edit it with $ sudo nano /etc/X11/xorg.conf

7. The assignment of the physical XF86MonBrightnessDown and XF86MonBrightnessUp keys is explained here for Xubuntu LTS or XFCE users.

8. Finally, reboot for these changes to take effect.

Finer-grained brightness control (all Ubuntu versions)

If brightness control happens to be too coarse, then make this additional adjustment.

Ubuntu LTS 16.04

To find out if you have integrated Intel video graphics, enter the following command:

$ ls /sys/class/backlight/
    intel_backlight  panasonic

At least intel_backlight should be mentioned, most probably in addition to an OEM name like for example panasonic, dell_backlight, etc.

If this is the case, proceed with creating the following file

$ sudo nano /usr/share/X11/xorg.conf.d/20-intel.conf

containing the following lines:

Section "Device"
        Identifier  "card0"
        Driver      "intel"
        Option      "Backlight"  "intel_backlight"
        BusID       "PCI:0:2:0"
EndSection

Reboot, and enjoy your backlight buttons! [Source]

Before Ubuntu LTS 16.04

Under (X)Ubuntu 14.04 LTS, brightness control on my Panasonic Toughbook CF-52 used to work fine by adding acpi_osi=Linux to the GRUB_CMDLINE_LINUX_DEFAULT= line in /etc/default/grub:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"

and issuing

$ sudo update-grub

after editing and before rebooting.

Another option is the application Redshift, which in addition to adjusting brightness can also adjust the color temperature to reduce the blue light at night.

Example usage: redshift -b 0.8 will set the brightness to 0.8 on a scale of 0 to 1

I use Lenovo Z570 and adding acpi_backlight=none to /etc/default/grub and updating grub worked nicely with Fn key.

In XFCE, you must enable "Handle display brightness keys" on XFCE Power Manager, under the General tab.

In 16.04 there is a package in the repos called "backlight-indicator" which will use your camera to set backlight (or not) and differentially set it for AC and/or battery usage.

I fixed this on my Asus UX303UB with a little difference:

• set to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=" (acpi_osi= did it for me, other options didn't work, got this from here). Of course after this run update-grub. This make function keys respond showing the Ubuntu notification, but doesn't really change brightness. One more thing taken from other answers:
• sudo nano /usr/share/X11/xorg.conf.d/20-intel.conf

with contents:

Section "Device"
    Identifier  "card0"
    Driver      "intel"
    Option      "Backlight"  "intel_backlight"
    BusID       "PCI:0:2:0"
EndSection

Now the only problem is to associate Fn+F7 to xset dpms force standby to turn off the screen.

In my case, the problem was because of some issue with my graphics drivers. Changing them to a stable version solved the issue.

I have a Panasonic CF-LX3, and it reports as having 2 brightness controls:

/sys/class/backlight/:
intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight
panasonic -> ../../devices/virtual/backlight/panasonic

But actually on this machine the first one works and the vendor one doesn't.

So adding acpi_brightness=video to GRUB_CMDLINE_LINUX_DEFAULT in /etc/defaults/grub helped.

(don't forget to update-grub)

After following the accepted answers, it still did not work in Ubuntu 18.04. Then, I changed /etc/default/grub to contain the line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=none"

Instead of:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

And run:

sudo update-grub

Then reboot. It worked perfectly.