Patterns have recognised utility in software engineering design and development processes. The body of knowledge of design and development of agent-based systems is now sufficiently mature to be usefully expressed as a pattern language. The essential elements of agent-based decomposition? autonomous agents, relationships between agents, and organisational structures-lend themselves to a pattern-based description. This paper describes an initiative currently underway at Agentis Software to mine, document and evolve a complete pattern language for agent-based systems. These patterns define recurring agent-based structures for both individual agents and multi-agent system architectures. This paper discusses the software engineering motivations for making this investment in patterns by describing the strategic and pragmatic motivations, the pattern mining process, how the patterns are used in the design of agent-based solution architectures, and our experiences in using and evolving the pattern language.