Source SDK will not launch

From Valve Developer Community

Problem: You launch the SDK from Steam, and a window comes up saying "Launching Source SDK." After a few seconds, the window goes away, and nothing happens beyond that. The SDK window itself never appears.

Solution:

1. Enter Steam and go to Tools.
2. Right-click on Source SDK.
3. Click on Properties.
4. Click on Set Launch options...
5. Enter the following in the box: -engine ep1
6. Click OK.
7. Click Close.

Source SDK should now work.

If this article didn't fix your problem, or your symptoms are slightly different, please post it at the discussion page.