Why? Dirty CDs can cause read errors and/or cause CDs to not work at all.

Procedure: Cleaning CDs and DVDs should be done with a cleaning kit but can also be done with a normal clean cotton cloth or shirt. When doing this with a clean cotton cloth or shirt, wipe against the tracks, starting from the middle of the CD or DVD and wiping towards the outer side as shown in the below picture. Never wipe with the tracks; doing so may put more scratches on the disc.

It is recommended when cleaning a CD that water is used. However, if the substance on a CD cannot be removed using water, pure alcohol can also be used.