Coagulopathy (hypo- and hypercoagulability) is common in the intensive care setting. Traditionally, coagulation potential has been assessed using plasma-based tests, such as the prothrombin time (PT) or the activated partial thromboplastin time (APTT); but these tests have repeatedly been shown to be poor predictors of bleeding and thrombosis [1]. Reliable prediction and/or diagnosis of clinically relevant clotting dysfunction and subsequent monitoring of effective treatment are important management issues in critically ill patients. Viscoelastic tests, such as TEG® or ROTEM®, are increasingly being used to predict coagulopathy and guide transfusion therapy. This chapter will briefly describe the mechanics of these tests, discuss their potential for use in the critical care setting and touch on the evidence that supports their use.