Quantitative modeling in behavior analysis might be traced to Thorndike’s ground-breaking observations of individual behavior, with mathematical modeling beginning later and proceeding through increasingly sophisticated models. Good models succinctly summarize key determinants of behavior, integrate a range of empirical findings, and guide application. Most mathematical models in behavior analysis engage in homogeneous reduction, in which a phenomenon is reduced parsimoniously to controlling variables acting at the same level at which observations are made. Dimensional analysis of models must show it to be consistent internally as well as with the behavioral measures it purports to explain. To illustrate these foundational concepts, the chapter then examines Shull’s analysis of the microstructure of behavior as a series of bouts and pauses. The model, its key parameters, and its applications in other scientific areas as well as in the clinic are examined.