Target-specific oral anticoagulants are approved for use for the prevention of stroke in atrial fibrillation and for the prevention and treatment of venous thrombosis without the need for laboratory monitoring. However, there are clinical settings in which laboratory measurement of anticoagulant effect is needed. These may include patients with life-threatening bleeding or those requiring emergency surgery, in the setting of renal or hepatic failure, or patients with thrombosis on therapy. This chapter reviews the use of laboratory testing to assess the anticoagulant effect of these drugs. In addition, because these drugs can interfere with other laboratory testing, available data on these interactions are presented.