Engine-driven generators are a primary source of electrical power in an aircraft. Generators can supply either direct or alternating current (DC or AC), as appropriate to the needs of an individual aircraft type. Motors (which can also be either DC or AC types) are fitted in aircraft in order to satisfy a wide range of needs. Generators and motors share many similarities and this chapter provides an introduction to their operating principles by looking in more detail at the construction and operation of these indispensable aircraft electrical components. The chapter also includes a brief introduction to three-phase AC supplies, including the theoretical and practical aspects of their generation and distribution.