The proceedings of the fourth Vienna Development MethodSymposium, VDM'91, are published here in two volumes.Previous VDM symposia were held in 1987 (LNCS 252), 1988(LNCS 328), and 1990 (LNCS 428).The VDM symposia have been organized by VDM Europe, formedin 1985 as an advisory board sponsored by the Commission ofthe European Communities. The VDM Europe working groupconsisted of researchers, software engineers, andprogrammers, allinterested in promoting the industrialusage of formal methods for software development. The fourthVDM symposium presented not only VDM but also a large numberof other methods for formal software development.Volume 1 contains conference contributions. It has fourparts: contributions of invited speakers, papers, projectreports, and tools demonstration abstracts. The emphasis ison methods and calculi for development, verification andverification tools support, experiences from doingdevelopments, and the associated theoretical problems.Volume 2 contains four introductory tutorials (on LARCH,Refinement Calculus, VDM, and RAISE) and four advancedtutorials (on ABEL, PROSPECTRA, The B Method, and TheStack). They present a comprehensive account of the state oftheart.