To date, general relativity has satisfied all experimental tests. However, proposals that may lead to a quantum theory of gravity (such as string theory and loop quantum gravity) generically predict violations of the weak equivalence principle in the 10 to 10 range. Currently envisioned tests of the weak equivalence principle are approaching a degree of sensitivity such that non-discovery of a violation would be just as profound a result as discovery of a violation. Non-discovery of equivalence principle violation in this range would suggest that gravity is so fundamentally different from other forces as to require a major reevaluation of current attempts to unify gravity with the other forces of nature. A positive detection, on the other hand, would provide a major guidepost towards unification.