Recurrent pregnancy loss (RPL) is defined as three or more consecutive spontaneous abortions. It affects about 1% of couples attempting pregnancy. Most of these pregnancy losses are unrecognized. Among the pregnancies that are clinically recognized, loss occurs in 15% before 20 weeks of gestation. Only about 50% of women with RPL have an identifiable etiology. Genetic abnormalities are responsible for RPL in 2~4% of these couples, most with balanced translocation. Luteal phase defect might be present in 25~30% of women with RPL; however, progesterone supplementation for miscarriage prevention has not been adequately studied. Uterine abnormalities might also result in RPL. Inherited thrombophilias appear to be associated with fetal death, and antenatal thromboprophylaxis might be beneficial for some women with an inherited thrombophilia. Antiphospholipid syndrome is known to cause RPL, and antenatal thromboprophylaxis reduces the risk of miscarriage. The evidence on alloimmune incompatibility as a cause of RPL is preliminary and no immunotherapy has been shown to be effective in the prevention of miscarriage. Recently a series of new factors that exhibit an association with RPL have been identified, and they might exhibit an additive or multiplicative effect on the RPL risk.