File Extensions BAK or BK File Format SUMMARY: BAK and BK files usually are backups of other documents or files.   The BAK (or BK) file extension normally represents files that are backup copies of other file. There is no 'standard' file format for BAK files. BAK files can represent virtually anything. If you're running out of hard drive space, BAK files can sometimes be safely removed if they are a couple of days old and you have closed down all running software. However, before you do so, you should consider what file the BAK file is a backup of; it may be beneficial to keep the file. Examine the filename as well as the date and time the file was created. If you have background processes running that make copies of certain files by creating BAK files, removing these files negates any usefulness of such processes. Windows users: NOTE that unless you have configured your computer with the file extension viewing tweak, it is possible that a filename that looks like it ends with ".bak", especially one attached to an e-mail message, actually has a hidden OTHER extension, meaning that if you double-click the file, it may actually run a computer virus! Since BAK files are supposed to be backup files, I would be VERY WARY of a BAK file sent via electronic mail!   Add: Del.icio.us | Digg | Furl | My Yahoo!   Discuss This Tip Print This Tip Get E-Mail When New Tips are Online Return to the File Extensions page.