In computing, SUBST is a command on the DOS, IBM OS/2, Microsoft Windows and ReactOS operating systems used for substituting paths on physical and logical drives as virtual drives.

In MS-DOS, the SUBST command was added with the release of MS-DOS 3.1. The command is similar to floating drives, a more general concept in operating systems of Digital Research origin, including CP/M-86 2.x, Personal CP/M-86 2.x, Concurrent DOS, Multiuser DOS, System Manager 7, REAL/32, as well as DOS Plus and DR DOS (up to 6.0). DR DOS 6.0 includes an implementation of the SUBST command. The command is also available in FreeDOS and PTS-DOS. The Windows SUBST command is available in supported versions of the command line interpreter cmd.exe. In Windows NT, SUBST uses DefineDosDevice() to create the disk mappings.

The JOIN command is the "opposite" of SUBST, because JOIN will take a drive letter and make it appear as a directory.

Some versions of MS-DOS COMMAND.COM support the undocumented internal TRUENAME command which can display the "true name" of a file, i.e. the fully qualified name with drive, path, and extension, which is found possibly by name only via the PATH environment variable, or through SUBST, JOIN and ASSIGN filesystem mappings.

This is the command syntax in Windows XP to associate a path with a drive letter:

This means that, for example, to map C:'s root to X:, the following command would be used at the command-line interface:

Upon doing this, a new drive called X: would appear under the My Computer virtual folder in Windows Explorer.

To unmap drive X: again, the following command needs to by typed at the command prompt: