The study of aPL antibodies and the APS has appealed to large numbers of investigators over the last decade. This accounts, in part, for the great degree of apparently contradictory data being published. What seems certain is that these antibodies are associated with thrombosis and recurrent pregnancy loss, and animal data suggest a direct role in pathogenesis. In vitro studies demonstrate that these antibodies have a variety of functional effects on the hemostatic system, giving further credence to an antibody role in thrombosis. Further studies will doubtless give better insight into the mechanisms of antibody action and this will provide a more rational basis for treatment.