This chapter provides a review of basic concepts in power systems and electrical circuits. These basic concepts provide the fundamental knowledge needed to carry out any design calculations for building electrical systems. First, a review of the basic characteristics of an electric system operating under direct current is provided to introduce basic properties of electricity. Then, a summary of alternating current properties are presented for both single- and three-phase systems commonly used in buildings. Throughout this chapter, several calculation examples are presented to further explain the basic principles of electrical circuits in buildings and show the applications of these principles to determine the characteristics and ratings of various equipment and loads.