The clinical classification of acute coronary syndromes without ST-segment elevation has long been a topic of debate. Braunwald's classification, however, is now widely adopted and permits to recognize the most severe forms of acute coronary syndromes. Risk stratification is essential and is based upon clinical and biological markers. Among them, recent and repeated anginal attacks, ST-segment modifications on admission electrocardiogram, and increased markers of myonecrosis (particularly increased troponine levels) are strong predictors of untoward outcome. These variables can be used to construct risk scores, among which the Timi risk score is the most widely used.