This book constitutes the refereed proceedings of the 11th International Conference of Z Users, ZUM '98, held in Berlin, Germany, in September 1998. The 28 revised and invited papers presented were carefully reviewed and selected for inclusion in the book; also included is a comprehensive Z bibliography and a summary of the frequently asked questions at two Z Internet sites. The papers are organized in sections on industrial issues, concurrency, tools, Z and HOL, safety-critical and real-time systems, semantic theory, theory and standards, reasoning and consistency issues, refinement, and object orientation.