This chapter discusses how, in essence, electromechanical rotating machines can be operated as generators or motors. The fact that rotating machines are called motors when they consume electrical energy or convert electrical energy into mechanical energy, work, or torque is explained. This chapter further discusses how rotating machines are referred to as generators when they produce electrical energy or power from mechanical energy or brake horsepower. This chapter describes how in practical applications, while direct current (DC) machines are almost always single-phase, alternating current (AC) machines can be single-phase or three-phase. This chapter explores fundamental operating principles and concepts associated with DC and AC motors and generators. The electromagnetic principles behind the operation of generators and motors are illustrated through simplified electrical diagrams. Basic principles and equations governing important and practical functions and operational parameters of motors and generators are introduced. Common calculations involving electric motors are illustrated. Concept of induction motor slip is explained and associated calculations are covered.