Throughout the book we have stressed the fact that PBC is compatible with one of the important viewpoints of systems theory that complicated systems are best thought of as being interconnections of simpler subsystems, each one of them being characterized by its dissipation properties. This aggregation procedure has three important implications. First, it is consistent with the dominating approaches for modeling and simulation based on some kind of network representation and energy flow. Second, it help us to think in terms of the structure of the system and to realize that sometimes the pattern of the interconnections is more important than the detailed behaviour of the components. Finally, it is indeed a design-oriented methodology which allows us to isolate the "free subsystems" — sensors and actuators.