Myoelectric pickups (electrodes and processors for detecting the signal that is recorded as an electromyogram) are the most important human–machine interface for controlling powered upper-extremity prostheses. This article presents a simple explanation of myoelectric signal acquisition and then discusses how these signals are used to control the small motors in electric hands, elbows, wrist rotators, and other similar equipment. The less-familiar switch-based and proportional position-sensing controls are also explained. A complete listing of the major suppliers and products available will aid in understanding a discussion of the criteria for using external power instead of, or along with, body power to control and activate prosthetic function.