Acute coronary syndrome includes instable angina pectoris, non-Q-wave infarction and, in current publications, usually also Q-wave infarction. The common pathological substrate is intracoronary plaque rupture with the formation of a thrombus. A careful work-up of the symptoms and their classification permits a presumptive diagnosis. This must then be confirmed by ECG without delay. On the basis of the clinical picture and the ECG, an acute myocardial infarction can be diagnosed unequivocally, and treatment initiated immediately. In these cases, laboratory findings merely confirm the diagnosis, and provide information on the size of the infarction and the success of reperfusion. In the event of an unclear ECG, rapid troponin tests may help to establish the diagnosis and monitoring of treatment, as well as providing prognostic information. While, formerly, a strict differentiation was made between instable angina pectoris, non-transmural and transmural infarctions, current pathophysiological knowledge makes the overarching term of acute coronary syndrome appear to be a more suitable term.