Antiphospholipid antibodies (aPL) are implicated in approximately a quarter of a million thrombotic events in the United States annually. These antibodies, anticardiolipin, anti-β2 glycoprotein I, and the lupus anticoagulant, can be found in healthy subjects with an increased prevalence in older populations, in children, in infections, and with malignancies. The risk of thrombosis is highest with multiple high-titre antibodies and in those in whom the lupus anticoagulant is detected. Factors such as hypertension and smoking also contribute to the rate of thrombosis. In patients with systemic lupus erythematosus (SLE), the lupus anticoagulant is the strongest predictor of thrombosis, with a 50% 20-year risk. The most common manifestation of antiphospholipid syndrome is deep venous thrombosis. Stroke is the most common arterial event, and aPL are implicated most frequently in those under the age of 50. There are conflicting data regarding pregnancy morbidity and aPL. Whilst pregnancy loss appears to occur at a higher rate with aPL, the data on other pregnancy morbidity, such as preeclampsia and the HELLP syndrome, have not consistently shown an association. In SLE, aPL are common, with the prevalence of anticardiolipin antibodies being 46% and that of the lupus anticoagulant 26%. Thus, aPL occur at a low frequency in the general population and are implicated in many thrombotic events. The risk of thrombosis increases with the antibody titre, number and presence of the lupus anticoagulant, which is of particular importance in the prediction of thrombosis in SLE.