© 2019 Elsevier B.V. In this chapter, we take a candid look at the practical application of simulation modeling. We suggest a strategy for model development that we have found helpful in traversing the pitfall-filled modeling landscape we depicted in Chapter 8. Although theoretically it is convenient to describe the modeling process as proceeding smoothly through the four phases described in Chapters 3–7 Chapter 3 Chapter 4 Chapter 5 chapter 6 Chapter 7, in practice we usually cycle through each of these phases several times. We seldom quantify the entire conceptual model before running simulations and evaluating model behavior. Rather, we usually construct a simple “running” model as quickly as possible and then expand it gradually through a series of small additions, each of which we carefully evaluate, until we have quantified the entire model.