With the advent of the coronary care unit and efficient arrhythmia detection and treatment, the prognosis of the patient admitted with acute myocardial infarction has substantially improved. Rarely is survival now limited by primary ventricular arrhythmias; instead the major limiting prognostic factor has become the extent of myocardial necrosis - the 'myocardial infarct size.' Recent studies have demonstrated that myocardial infarct size, assessed clinically, correlates with duration of survival, with the extent of left ventricular dysfunction, and with the frequency of serious ventricular arrhythmias. Thus, knowledge of myocardial infarct size is relevant clinically in order to estimate the patient's prognosis. Because of the recent availability of methods which have the potential for limiting myocardial infarct size, additional incentive has appeared for precisely measuring infarct size clinically. Indeed, much of the controversy surrounding the topic of infarct size limitation stems from the difficulty of reliably assessing infarct size in patients in order to prove that a given intervention is effective. Clinical techniques available for estimating the extent of myocardial necrosis are pathologic, angiographic, enzymatic, electrocardiographic, and scintigraphic. Each of these methods employs independent criteria for evaluating myocardial infarct size. Unfortunately, none of the methods is uniformly reliable or accurate. This survey will examine each technique briefly in an attempt to point out the relative strengths and weaknesses of each. The discussion will be limited to those techniques which have received application, not only in experimental animals, but also in patients.