This book constitutes, together with its compagnion LNCS 1708, the refereed proceedings of the World Congress on Formal Methods in the Development of Computing Systems, FM'99, held in Toulouse, France in September 1999. The 92 revised full papers presented in the two volumes were carefully reviewed and selected from a total of 259 paper submissions from 35 different countries. Also included are 15 abstracts describing work in progress and industrial applications. The papers are organized in topical sections. This volume contains the following sections: foundations of system specification (IFIP WG 1.3); European Theory and Practice of Software (ETAPS); program verification; integration of notation and techniques; formal description of programming concepts (IFIP WG 2.2); open information systems; co-design; refinement; safety; OBJ/Cafe OBJ/Maude; Abstract State Machines (ASM) and Algebraic Methods in Software Technology (AMAST); avionics; works-in-progress; industrial experience.