This book assesses the state of the art of agent-based approaches as a software engineering paradigm. The 15 revised full papers presented together with an invited article were carefully selected from 43 submissions during two rounds of reviewing and improvement for the 4th International Workshop on Agent-Oriented Software Engineering, AOSE 2003, held in Melbourne, Australia, in July during AAMAS 2003. The papers address all current issues in the field of software agents and multi-agent systems relevant for software engineering; they are organized in topical sections on - modeling agents and multi-agent systems - methodologies and tools - patterns, architectures, and reuse - roles and organizations.