Chapter 2 focused on steady-state models to represent power system elements for the so called static analysis studies. We now turn our attention to models for transient or dynamic operational studies. This chapter describes some system models for analytical purposes. A model of an excitation system is studied in Section 3.1 and Section 3.2 gives a discussion of a model of the prime mover and governor system. Dynamic load models are discussed in Section 3.3 to conclude the chapter.