Mention the word relativity and everyone thinks 'Einstein'. Less well known is the fact that Einstein invented two theories of relativity, his 'special' theory (1905) which tells what happens to bodies moving at very high speeds, and his 'general' theory (1916) which is about gravity. The NASA-Stanford Gravity Probe B Mission (GP-B) launched from Vandenberg Air Force Base in April 2004, tests with extreme accuracy two extraordinary predictions of general relativity. Join Professor Francis Everitt for a lively explanation of GP-B's development.