Evoked potentials (EPs) are the electrical signals produced by the nervous system in response to external stimulation. The term was initially restricted to sensory EPs, elicited most often by visual, auditory, or somatosensory stimulation, but has more recently been expanded to include motor evoked potentials, which are signals generated in neural tissue (such as the spinal cord) or in muscles following electrical or magnetic stimulation of the descending motor tracts in the brain. The recording electrodes placed on the subject pick up a variety of other signals in addition to the desired EP. These other extraneural signals, which may be labeled "artifacts", are the subjects of this chapter. Some of them are of physiologic origin, such as electrocardiogram, electroencephalogram, the electrooculogram, and electromyographic activity from muscles near the recording electrodes. Others, such as the electrical stimulus artifact, are related to the stimuli used to elicit the EPs.