MAP recordings have been at the cradle of cardiac electrophysiology but only recently, through safer and simpler technology, have gained wider access to clinical electrophysiology. In contrast to conventional electrode catheter recordings, MAP recording devices provide precise information not only of the local activation time but of the entire local repolarization time course as well. Although the MAP does not reflect the absolute amplitude or upstroke velocity of transmembrane action potentials, it delivers highly accurate information on the action potential duration and configuration, including early afterdepolarizations as well as relative changes in transmembrane diastolic and systolic potential changes. Based on available data, the MAP probably reflects the transmembrane voltage of cells within a few millimeters of the exploring electrode. MAPs can be recorded by catheter technique from the endocardial surface and by special probes from the epicardium in the operating room. The contact electrode technique is preferable over suction electrodes because it is safer and simpler to use in patients and because it produces more stable, longer-lasting signals. A modified contact MAP catheter incorporates pacing electrodes and permits simultaneous assessment of action potential duration and refractoriness. This not only facilitates the use of MAP catheters in routine electrophysiological studies but also is important for assessing the voltage-independent effects of antiarrhythmic drugs on refractoriness. MAP recordings offer the opportunity to study, in the in situ heart, a variety of pertinent electrophysiological phenomena including, for example, effects of cycle length changes and antiarrhythmic drugs on action potential duration or the role of afterdepolarizations in the genesis of triggered arrhythmias. Due to vigorous heart beating, movement artifacts may occur and need to be distinguished from true abnormalities in the action potential time course. With these limitations in mind, MAP recordings are a valuable addition to clinical electrophysiological studies.