Fn+F5,F6 keys to adjust brightness doesn't work

I have a Asus X551MA, here is what worked for me.

Update Grub;

1. Open terminal with Ctrl+Alt+T

2. type in terminal;
   gksudo gedit /etc/default/grub

3. Change;
   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
   to
   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pcie_aspm=force acpi_osi="

4. Save and Exit gedit

5. type in terminal;
   sudo update-grub

6. Restart your computer; (Don't forget to bookmark this page)
   sudo shutdown -r now
   This should fix your issue, if not keep reading.

Create 20-intel.conf; Intel GFX card only!

1. Open terminal with Ctrl+Alt+T

2. type in terminal;
   cd /usr/share/X11/xorg.conf.d

3. now;
   sudo touch 20-intel.conf

4. then type;
   gksudo gedit 20-intel.conf

5. Copy & Paste the following lines into the document;

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

6. Save and Exit the document.

7. Restart your computer;
   sudo shutdown -r now
   This should work, if not keep reading.

Install 3.16 kernel for 14.04 LTS;

1. Update;
   sudo apt-get update

2. Install;
   sudo apt-get install linux-image-generic-lts-utopic

That's all I got, sorry if this is incompatible with your set-up as there may be additional steps required, and I prefer to refrain from giving blind advice. I do recommend upgrading the kernel however, as this may be the solution you need, if you are using Ubuntu 14.10 or later there is no need to upgrade the kernel.

I have an ASUS UX305FA zenbook and the above suggestions re: messing around with /etc/default/grub ddidn't work for me.

This worked for me: https://github.com/thezerobit/asus-zenbook-ux305fa

Install xbacklight

sudo apt-get install xbacklight

In Keyboard settings, set up custom shortcuts to increase or decrease backlight by calling xbacklight -inc 10 or xbacklight -dec 10.

Edit: I own this laptop too and the xbacklight workaround does work. you should just not use the fn + f5/f6 buttons but use other keys, mine are ctrl + f3/f4. Also if i decrease too much, the screen will go entirely black..