One of the central issues in software and system engineering over the last couple of decades has been to develop languages, methods and tools for the reliable construction of reactive systems, those whose complexity stems not necessarily from complicated computation but from complicated reactivity over time (Harel and Pnueli, 1985). Such systems are usually highly concurrent and timeintensive, and can exhibit hybrid behavior that is predominantly discrete in nature but has continuous and stochastic aspects too. The structure of these systems consists of many interacting, often distributed, components. Very often the structure itself is dynamic, with its components being repeatedly created and destroyed during the system’s life span. The heart of the problem is the need for good approaches to modeling and analyzing the dynamic behavior of such systems.