To facilitate the design of large-scale, self-adaptive systems, we have developed the Port-Based Adaptable Agent Architecture. This distributed, multiagent architecture allows systems to be created with the flexibility and modularity required for the rapid construction of software systems that analyze and dynamically modify themselves to improve performance. This architecture provides user-level access to the three forms of software adaptability: parametric fine tuning, algorithmic change, and code mobility. In this paper, we present the architecture, describe port-based agents, and outline several applications where this flexible architecture has proven useful.