This chapter argues that modeling is at the core of every service engineering method. Modeling not only allows us to understand business services but, what is equally important, to transform them into software-realized services. In general, models provide abstractions of a physical system that allow engineers to reason about that system while ignoring irrelevant details and focusing on relevant ones (Brown, 2004). All forms of engineering and science rely on models to understand complex, real-world systems. Models are used to predict system properties, reason about how changes in some parts of it will affect the rest of a system, and communicate key system characteristics to various stakeholders. The models may be developed as a mock-up or blueprint prior to implementing the physical system, or they may be derived from an existing system or a system in development, as an aid to understanding its behavior.