This book constitutes the thoroughly refereed post-proceedings of the First International Workshop on Formal Approaches to Agent-Based Systems, FAABS 2000, held in Greenbelt, MD, USA, in April 2000. The 22 revised full papers presented together with 13 posters and two panel discussion reports were carefully reviewed and improved for inclusion in the book. The papers are organized in topical sections on verifying agents' mental states, synthesizing agents initially, frameworks and formalizations, modeling and execution, inter-agent communication, and adaptive agents.