Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent vascular thrombosis and loss of pregnancy in association with the presence of antiphospholipid antibodies (APA) detectable as lupus anticoagulants, anticardiolipin antibodies or anti-beta2 glycoprotein I antibodies. The pathophysiological importance of APA in APS is accepted, however, the mechanisms leading to thrombosis are likely to be multifactorial and are so far unclear. Without a prior thrombosis, the risk of developing a new thrombosis in healthy patients with APA is slightly increased (<1% per year). However, the risk of a recurrent thrombosis increases considerably (>10% per year) in patients with a history of thrombosis without anticoagulation. The careful and correct identification of patients with APS is important because prophylactic anticoagulation can reduce the risk of recurrent thrombotic events, and during pregnancy can improve fetal and maternal outcome.