  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.
