Piezoelectric ultrasonic vibrations provide rotary or linear motion in ultrasonic motors by means of friction. A larger force/torque is obtained from ultrasonic motors than from conventional electromagnetic devices of the same weight. The basic principles of linear and rotary motors that consist of two orthogonal vibrations are introduced. These are categorized as traveling wave and hybrid transducer ultrasonic motors. Their fundamental concepts are explained together with equivalent circuit modeling, motor performance, and driving circuits.