Unstable angina pectoris is a clinical syndrome with multiple underlying pathophysiologic mechanisms. This presentation is concerned with primary angina pectoris exclusively. In the majority of cases a rupture of an atherosclerotic plaque and an intracoronary thrombus are responsible for instable angina. The practitioner's role is to identify those patients who will develop complications with the aid of clinical parameters. Prinzmetal's angina is also instable, occurs at rest and leads to ST-segment elevation. It is most likely due to coronary spasm, developing in disease-free and atherosclerotic coronary segments alike. This variant of unstable angina is treated most successfully with calcium antagonists. The recognition of the responsible pathophysiologic mechanism permits adjustment of treatment of every patient taking into consideration the seriousness of his prognosis.