This chapter describes some system models for analysis purposes and introduces concepts of power expressed as active, reactive, and apparent, followed by a brief review of three-phase systems and the per unit system representations. Electrical power systems vary in size, topography, and structural components. However, the overall system can be divided into three subsystems: generation, transmission, and distribution. The power industry in the United States has been engaged in a changing business environment for quite some time now, moving away from a centrally planned system to one in which players operate in a decentralized fashion with little knowledge of the full state of the network, and where decision making is likely to be market driven rather than based on technical considerations. The electrical power systems specialist is, in many instances, more concerned with electrical power in the circuit rather than current.