A critical review is given of the tests available for the assessment of the action of anticoagulants, such as heparins, oral anticoagulants and direct thrombin inhibitors, in patients under antithrombotic therapy. The principle of action and the performance of the thromboplastin time (PT), the activated partial thromboplastin time (aPTT), the whole blood clotting time, the thrombin time, the ecarin clotting time and the endogenous thrombin potential (ETP) is discussed, as well as the evidence behind the accepted therapeutic ranges. The two most common tests, PT and aPTT, respond in an essentially different way to clinically effective anticoagulation with heparin and with oral anticoagulants. This means that they covariate with, but do not themselves represent the essential parameter influenced by anticoagulation. The experimental basis for the widely accepted two times prolongation of the aPTT as an indicator for adequate anticoagulation is shown to be meagre in the case of unfractionated heparin and lacking for the other anticoagulants. Common sources for error in the interpretation of anti-factor Xa- and anti-thrombin activity of heparins are indicated. Extensive experience with new tests like the ecarin clotting time and the ETP is still lacking. On the basis of preliminary data and in view of the importance of the enzymatic action of thrombin in the pathogenesis of thrombosis, the ETP is considered a possible candidate for a common parameter to assess different types of anticoagulants.