Thrombelastography is a 'near patient' test of coagulation. It is easy to perform and can provide information on a patient's coagulation status within 30 min. Despite more than 25 years of clinical experience, however, several basic questions relating thromboelastograph (TEG) parameters to standard coagulation tests remain unanswered, and the value of the TEG is established only in the setting of orthotopic liver transplantation and cardiopulmonary bypass surgery. This review will focus on the principles and practise of the TEG(R), and data supporting the current accepted uses. Potential future uses will also be discussed including evaluation of hypercoagulable states, and investigating the mechanism of coagulopathies due to drugs or disease that standard tests have failed to unravel.