Anticoagulation during renal replacement therapy is recommended to avoid thrombosis of the filter devices and to maintain the blood flow. However, in the case of multiorgan failure and sepsis, an imminent bleeding complication in patients with acute renal failure may cause the need for an extracorporeal circulation without anticoagulation. The most common drug used in renal replacement therapy is the unfractionated heparin (UFH). With low molecular weight heparin (LMWH) good experiences are reported, too. Based on the level of evidence from clinical studies plasma measurement of heparin is indispensable for patients with renal insufficiency. The activated whole blood clotting time (ACT), the activated partial thromboplastin time (aPTT), and the determination of the anti-factor Xa activity (anti Xa) with chromogenic substrates are available as routine as well as as point-of-care tests. To monitor plasma levels of LMWH the anti Xa assay serves exclusively as a suitable monitoring. The anti Xa assay using chromogenic substrates is the most specific and valid one for monitoring heparin therapy. In lack of large controlled studies for the anticoagulation therapy and its monitoring with the anti Xa test in acute renal failure, the current experiences are based on the results of chronic renal replacement therapy.