Ten Years of the Fifth Force
  The suggestion in 1986 of a possible gravity-like ``fifth" fundamental force renewed interest in the question of whether new macroscopic forces are present in nature. Such forces are predicted in many theories which unify gravity with the other known forces, and their presence can be detected by searching for apparent deviations from the predictions of Newtonian gravity. We review the phenomenology behind searches for a ``fifth force", and present a summary of the existing experimental constraints. 
Ephraim Fischbach and Carrick Talmadge