Reliable models of the gas exchange process in two-stroke cycle engines are a valuable tool to the engine designer. Because there is no one class of model that is capable of predicting the scavenging process in any type of scavenging system for the full range of cylinder geometries and operating parameters of interest, this chapter reviews the available scavenging models and illustrates their application. It classifies these models into three main categories: simple single-phase one- or two-zone models, multizone multiphase models, and multidimensional models. The two classic single-phase, one- or two-zone models, the pure-displacement and the perfect-mixing models, are simple and easy to use. The pure-displacement model, the upper bound for the charging efficiency, is considered to be the ideal scavenging process. It is a single-phase two-zone model. The perfect-mixing model gives the lower bound for the charging efficiency. It is a single-zone single-phase model.