The purpose of this chapter is to introduce students to a few simple rate theory models for describing phase transformations due to radiation damage. More comprehensive reviews have been provided elsewhere (see, e.g., a recent review by Weber [1]). This chapter introduces the reader to the concepts underlying the most oft-cited models. Most of the models to be presented here were initially developed to describe amorphization phase transformations. However, the models are more general to the extent that they are also viable to describe irradiation induced crystal-to-crystal solid state phase transformations.