This book presents the refereed proceedings of the International Workshop on Mathematical Methods, Models, and Architectures for Network Security Systems, MMM-ACNS 2001, held in St. Petersburg in May 2001. The 24 revised full papers presented together with five invited contributions were carefully reviewed and selected from 36 submissions. The papers are organized in topical sections on network security systems: foundations, models and architectures; intrusion detection: foundations and models; access control, authentication, and authorization; and cryptography and steganography: mathematical basis, protocols, and applied methods.