This book constitutes the refereed proceedings of the 12th International Conference on Concurrency Theory, CONCUR 2001, held in Aalborg, Denmark in August 2001. The 32 revised full papers presented together with six invited contributions were carefully reviewed and selected from 78 submissions. The papers are organized in topical sections on mobility, probabilistic systems, model checking, process algebra, unfoldings and prefixes, logic and compositionality, and games.