Surface electromyography (EMG) is a complex signal. This article will review issues of instrumentation, cross-talk, signal quantification, and anatomic interpretation. A surface EMG signal represents a random summation of action potentials propagating from many motor units which are activated during a particular movement. Despite limitations due to complex anatomical and physiological constraints, surface EMG offers valuable information concerning the timing of muscular activity and its relative intensity. A sophisticated understanding of the anatomic and technical issues in surface EMG is needed to appropriately analyze and interpret these complex signals.