This chapter proposes the adaptive agent model (AAM) for agent-oriented system development. In AAM, requirements can be transformed into externalised business rules. These rules represent agent behaviours, and collaboration between agents using the rules can be modelled using extended UML diagrams. Specifically, a UML structural model and a behavioural model are employed. XML is used to further specify the rules. The XML-based rules are subsequently translated by the agents. The UML diagrams and XML specification can both be edited at any time, the newly specified behaviours being available to the agent system immediately. An illustrative example is used to show how AAM is deployed, demonstrating adaptation of inter-agent collaboration, intra-agent behaviours, and agent ontologies. With AAM, there is no need to recode and regenerate the agent system when change occurs. Rather, the system model is easily configured by users and agents will always get up-to-date rules to execute at run-time.