The explosive growth of application areas such as e-commerce, enterprise resource planning, and mobile computing has profoundly and irreversibly changed our views on software systems. Nowadays, software is to be based on open architectures that continuously evolve to accommodate new components. Software must also operate on different platforms, without recompilation, and with minimal assumptions about its operating environment and its users. Furthermore, software must be robust and autonomous, capable of serving a naive user with a minimum of overhead and interference. Agent concepts hold great promises for responding to the new realities of software systems. This book constitutes the thoroughly refereed post-proceedings of the 5th International Workshop on Agent-Oriented Software Engineering, AOSE 2004, held in New York, NY, USA in July 2004 as part of AAMAS 2004. The 15 revised full papers were carefully selected from 52 submissions during two rounds of reviewing and improvement. The papers are organized in topical sections on modeling, design, and reuse and platforms.