Files with the extension .FLS are software flash files for use with certain Nokia mobile phones. The purpose of 'Flash' or rather the purpose of 'Flashing' is to change a mobile phones operating system or firmware.

A mobile phones operating system is not stored on a hard disk but is held on 'Flash memory' a non-volatile computer memory that can be electrically erased and reprogrammed.

.FLS files format may be specific to the software used to create a specific version of the .FLS file. Use of incorrect software may result in damage to the operating system.