Two key issues in building multi-agent systems concern their scalability and engineering open systems. We offer solutions to these potential problems by introducing a lifecycle for models of large multi-agent systems. Our proposal connects a model for the collective analysis of agent systems with an individual-based model. This approach leads on to a virtuous cycle in which individual behaviours can be mapped on to global models and vice-versa. We illustrate our approach with a formal example but relatively easy for engineers to follow and adapt.