There are three different aspects in the etiological diagnosis of hemostasis disorders: (1) primary hemostasis evaluation; (2) coagulation evaluation; and, (3) thrombosis evaluation. In every case, a look at the medical history is an essential step before proceeding to biological investigations. Bleeding time is the first step of the primary hemostasis evaluation; prolonged drug use (particularly salicylates) and thrombocytopenia must be first considered; only then will the Von Willebrand factor and platelet functions be studied. Coagulation evaluation first requires the study of the overall coagulation tests (Quicks test, partial thromboplastin time test and thrombin time test); determination of the different plasma coagulation factors and search for a circulating anticoagulant will be performed secondarily, allowing differentiation between the different acquired or constitutional coagulation disorders. Thrombosis evaluation must first consider a local or general inciting factor before looking for anti-phospholipid antibodies, an acquired protein C or S deficiency or a constitutional hemostasis disorder.