VMT is an extension found in files used by the game engine called Valve Source. VMT is a text file that defines a 2D surface. It is used to store information related to character animation, in-game physics and rendering. Each of these files also has a specific name of the cardboard and the parameters needed to visualize the surfaces and their shading.

The Source Engine was developed by Valve Corporation and was first used in the production of Half-Life 2.