Talk at Nato ASI, Cargese 1997, PUPT-1752 , hep-th/9712156

An overview over effects of D-instantons in ten dimensional IIB superstring theory is given, including the supergravity instanton solution, instanton induced effective interaction vertices, the conjectured SL(2, Z) invariant completion of such terms and the connection of such terms to a one loop calculation in eleven dimensional supergravity.