The study of antiphospholipid (aPL) antibodies has been greatly developed in recent years and conclusive evidence now exists concerning the correlation between aPL and clinical signs such as thrombosis, thrombocytopenia, abortion, and fetal loss. Several hypotheses have been put forward concerning the pathogenic mechanism of aPL, but none has received final confirmation from experimental data. Many studies have been devoted to characterizing the antigens recognized by the different aPL autoantibodies and to a cofactor involved in the binding of autoantibodies and phospholipids; this cofactor has been identified as an apolipoprotein, the beta 2 glycoprotein I (beta 2GPI) or APO-H. Direct evidence now exists which suggests that both the beta 2GPI and the phospholipid comprise the epitope to which aPL are directed. On the other hand anti-beta 2GPI antibodies have been identified in sera of patients suffering from SLE and primary Antiphospholipid Syndrome. beta 2GPI is normally present in human plasma/serum and possesses numerous inhibitory functions in multiple coagulation pathways. The amino acid sequence of beta 2GPI has been identified and found to consist of five repeating units that belong to the complement control protein (CCP) superfamily. This development of knowledge related to aPL has followed three steps respectively: 1. the standardization of the techniques of detection: 2. identification of the clinical signs related to the autoantibodies: and finally 3. the discovery of a new player, the beta 2GPI cofactor.