The neurophysiologic evaluation of patients with possible or proven paroxysmal disorders is no longer limited to routine laboratory EEGs or intensive inpatient monitoring. Expanded temporal sampling of the EEG, which increases the probability of documenting, characterizing, and quantitating the electrographic manifestations of these illnesses, is now available on a portable, outpatient, and less cumbersome inpatient basis by means of ambulatory cassette recordings. Ambulatory EEG has evolved from clinical need and from new technology that has provided small multichannel tape recorders, miniature preamplifiers, and rapid video/audio playback units. Clinical series and controlled trials have confirmed the usefulness of cassette EEG monitoring in a wide range of neurologic disorders, particularly epilepsy. Ambulatory EEG diagnostic yields have been shown to be superior to routine laboratory studies and nearly as good as inpatient telemetry evaluations. The role of cassette recordings in clinical EEG is being redefined as new applications are established.