Source SDK will not launch

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: Oops: the 'solution' that was on this page hasn't worked for the best part of a year!

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