An electric machine is an electromechanical device used for energy conversion from electrical to mechanical and vice versa. The machine can also process the energy flow in the reverse direction during regeneration. After starting with the principles of electromechanical energy conversion, the various types of electric machines and their operating principles are discussed in this chapter. The active materials for energy conversion and passive materials for frame and support structures are also presented. Although DC machines were used in a number of early electric vehicle prototypes, now only AC electric machines are used in EV/HEVs. Among the AC machines, the induction motors and the permanent magnet (PM) synchronous motors are primarily used as electric traction motors. The interior permanent magnet (IPM) synchronous motors utilizing neodymium-iron-boron (NdFeB) magnets became the design choice of traction electric machines primarily due to the unparalleled power/torque density and efficiency that can be achieved with these machines to meet the demanding requirements of an electric traction drivetrain. The electric machine types using the reluctance principle for torque production are also discussed since some of them have good potentials for use in electric vehicles. A traction IPM machine design example is included in this chapter as well.