This chapter focuses on the primary antiphospholipid syndrome, that is, in the absence of systemic lupus erythematodes. Women with thrombophilia have an increased baseline risk of venous thromboembolism. In antiphospholipid syndrome, lupus anticoagulant is more strongly related to venous thrombosis and pregnancy complications than antibodies against phospholipids. The chapter reviews the evidence regarding potential clinical implications of acquired and inherited thrombophilia for both venous thromboembolism and for pregnancy failure. Treatment guidelines vary with regard to the administration of heparin for antiphospholipid syndrome and recurrent miscarriage. For women with antiphospholipid syndrome, the evidence regarding the efficacy of aspirin with or without the addition of low-molecular-weight heparin is not solid, whereas two small trials have shown a clear benefit of unfractionated heparin. For women with inherited thrombophilia, low-molecular-weight heparin to prevent pregnancy loss is definitely experimental as solid evidence is not yet available.