  It is more important than ever to push experimental tests of gravitational
theory to the limits of existing technology in both range and sensitivity. This
brief review focuses on spin-based tests of General Relativity and their
implications for alternative, mostly non-metric theories of gravity motivated
by the challenge of unification with the Standard Model of particle physics.
The successful detection of geodetic precession and frame-dragging by Gravity
Probe B places new constraints on a number of these theories, and increases our
confidence in the theoretical mechanisms underpinning current ideas in
astrophysics and cosmology.
