The Source SDK (or Source software development kit) is a collection of tools compiled by Valve Software that is used to create levels or full-on Half-Life 2 modifications. It can also be used by developers to create a new game using the source game engine.

The main tools that are included in the Source SDK are:

• Valve Hammer Editor
  Formerly known as Worldcraft this is Valves level design editor.
  
  
• Valve Model Viewer
  The Model Viewer can be used to view models and their corresponding animations, attachment points, bones, etc. It can also be used to view props, weapons, buildings, and just about any other 3D object.
  
  
• Valve FacePoser
  This tool used to access Valve Source Engine facial animations and choreography systems. You can edit facial expressions, gestures and movements for characters and lip synch speech.
  

The Source SDK is available to all owners of Half-Life 2 (or other games run using the Source engine, such as Counter Strike: Source or Day of Defeat: Source) through Steam. It is selectable under "Tools" in the "Play Games" menu, and once selected, all the necessary files will be downloaded if they are not already on the user's computer.