Antiphospholipid syndrome (APLS) is characterized by recurrent thrombosis and/or pregnancy loss associated with persistently elevated levels of autoantibodies to phospholipids or plasma proteins. APLS occurs both as a primary disease and as a condition secondary to lupus and other autoimmune diseases. Awareness of the polymorphic presentations of APLS is vital given the risk of life-threatening thrombosis associated with this condition. Anticoagulant therapy is effective in preventing recurrent thrombosis. New biological markers have been developed, including autoantibodies to beta-2-glycoprotein 1 (anti-β2GP1), phosphatidylethanolamine (PE), and annexin-V. These autoantibodies are not listed in the new diagnostic criteria set (Sapporo, 1999) because their usefulness needs to be validated and their detection standardized. Nevertheless, they may be helpful as part of a biological evaluation strategy for APLS. Progress can be expected in the near future, particularly regarding prediction of thrombosis. Although no marker predicts thrombosis with complete certainty, persistent high titers of the IgG variants of one or more antiphospholipid antibodies (particularly circulating anticoagulant and anti-β2GP1) indicate a high risk of thrombosis. Evaluation of other thrombogenic factors such as inherited thrombophilia is probably useful in patients with APLS. Improvements in our ability to predict the risk of thrombosis may lead to novel strategies for prevention and treatment. New drugs such as complement factor (C3) inhibitors are being developed.