This book constitutes the refereed proceedings of the Third International Conference of Z and B Users, ZB 2003, held in Turku, Finland in June 2003.The 28 revised full papers presented together with 3 invited papers were carefully reviewed and selected for inclusion in the book. The book documents the recent advances for the Z formal specification notation and for the B method, spanning the full scope from foundational, theoretical, and methodological issues to advanced applications, tools, and case studies.