  Evidence in favor of $SL(2,Z)$ S-duality in $N=4$ supersymmetric Yang-Mills
theories in four dimensions and with general compact, simple gauge groups is
presented. (Contribution to the Proceedings of the Strings '95 conference,
March 13-18, 1995, USC, and the Proceedings of the Trieste Conference on
S-Duality and Mirror Symmetry, June 5-9, 1995.)
