This book constitutes, together with its compagnion LNCS 1709, 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. Besides five invited papers this volume contains the following sections: integration into the development process; software architecture; European Association for Theoretical Computer Science (EATCS); model checking; the B method; composition and synthesis; telecommunications; security; object-orientation; testing.