This book constitutes the refereed proceedings of the 6th International Conference on Formal Engineering Methods, ICFEM 2004, held in Seattle, WA, USA in November 2004.The 30 revised full papers presented together with abstracts of 9 invited contributions were carefully reviewed and selected from 110 submissions. The papers address all current issues in formal methods and their applications in software engineering, ranging from theoretical and methodological aspects to applications in various domains.