This book constitutes the refereed proceedings of the 15 IFIP International Conference on Testing of Communicating Systems, TestCom 2003, held in Sophia Antipolis, France in May 2003. The 19 revised full papers presented together with three invited contributions were carefully reviewed and selected from 53 submissions. The papers are organized in topical section on next generation networks, IP and UMTS; TTCN-3; automata-based test methodology; and test design, tools, and methodology.