The ongoing attempts to unify the known fundamental forces have led to the suggestion that there may exist new gravity-like forces in nature. These would arise from the exchange of new light bosonic quanta among the constituents of ordinary matter, and would produce apparent deviations from the predictions of Newtonian gravity. The suggestion of such a "fifth force" in 1986 helped to stimulate a large number of new experiments to search for weak long-range forces. This talk will review both the theoretical motivation for such new forces, and the experimental results that have been obtained to date.

More recently newer string-inspired theories have suggested the presence of additional macroscopic forces acting over sub-millimeter distances.Detecting such forces presents special challenges-both theoretical and experimental-for reasons that I will discuss.