CHKDSK (short for "check disk") is a command on computers running DOS, OS/2 and Microsoft Windows operating systems that displays the file system integrity status of hard disks and floppy disk and can fix logical file system errors. It is similar to the fsck command in Unix.

The command is implemented as an executable program file, CHKDSK.EXE. Different operating systems all use the same filename, but the files are not necessarily compatible between operating systems.

On computers running NT-based versions of Windows, CHKDSK can also check the disk surface for physical errors or bad sectors, a task previously done by SCANDISK. This version of CHKDSK can also handle some physical errors and recover data that is still readable.

CHKDSK can be run from the Windows Shell, the Windows Command Prompt or the Windows Recovery Console. One option for CHKDSK is the use of the Command-line /R parameter, which allows the program to repair damage it finds on the hard drive.

Conducting a CHKDSK can take some time, especially if using the /R parameter, and the results are often not visible, for various reasons. The results of a CHKDSK conducted on restart using Windows 2000 or later operating systems are written to the Application Log, with a "Source" name of Wininit or Winlogon and can be viewed with the Event Viewer. On the Windows 7 operating system, the events have a "Source" name of "Chkdsk"