In the past decade noninvasive, nontraumatic methods of monitoring parameters of physiological function have been investigated with increasing interest. Of the various parameters of cardiac function, only blood pressure and the electrical activity of the myocardium have tradionally been accessible noninvasively, and only the latter in a continuous fashion. Several invasive methods exist for measurement of myocardial contractility, cardiac output, the degree of heart failure, etc., and a few for watching some of these factors noninvasively, but not, in general, non- invasively, conveniently, comfortably, and continuously. Monitoring the changing thoracic electrical impedance is a technique which satisfies all these criteria. Some of the theory of biological impedance change is discussed, and some of the drawbacks are pointed out. A survey of the literature of the last decade is presented, illustrating the fact that many investigators have consistently obtained good correlation with 'standard' invasive techniques of measurement, while others have had variable results. Some of the unexplored areas in this field are discussed and directions for future research suggested.