Active objects are concurrent, active entities based on the object-oriented paradigm. We present a model for understanding active objects based on the remote procedure call, and on the notion of activities, which capture a single-thread flow of control between objects. We also present simple mechanisms for creating activities, interleaving and delaying activities, and for constructing atomic actions and concurrent subactivities. We show how these mechanisms can be used to capture very general forms of triggering. Our model for active objects, and the mechanisms for manipulating activities are embedded in Hybrid, a concuuent, object-oriented language. The model is also useful for understanding and dealing with deadlock in such systems.