As we see from the brief history of system dynamics presented in Chapter 1, much of the emphasis of system dynamics centers around the use of models and modeling techniques to study complex policy issues. As we proceed to outline system dynamics concepts, it is important that we step back briefly to consider the question, Why model at all? From observation, we know that modeling is something that all human beings do, and for a variety of reasons. However, the importance of understanding the distinction and application of models is less obvious. In this chapter, we continue to outline the basic premise and philosophy of system dynamics by addressing this fundamental question.