Imagine trying to explain how your favorite online store works—from the moment you click “buy” to the package arriving at your door—without drawing a single picture. Hard, right? Process modeling gives us a set of simple, visual tools to map out exactly what a system does with data and how it talks to the outside world. In this chapter, we’ll learn to draw and interpret data flow diagrams (DFDs), the most widely used tool for seeing the big picture of any information system.