Approximately 60 million women use the intrauterine device (IUD) worldwide; however, owing primarily to nonmedical reasons, the IUD is far less popular in the United States. Although the contraceptive mechanism of action is unknown, it appears that spermicidal activity may be important. Overall, the efficacy of the copper devices is quite good, such that the overall lifespan can probably be extended. Possible pelvic infection remains the greatest potential risk, although in properly selected women the risk is quite low. Use of prophylactic antibiotics at the time of insertion may offer additional protection against this risk. Although IUD users may have more nonspecific vaginal inflammation than do other women, the clinical significance is probably limited. Further, users do not appear to have elevated risks for cervical infections. Although menometrorrhagia persists as a potential problem, the mechanism for such bleeding is not well understood. Finally, the retroflexed uterine position does not appear to increase the risk of abnormal outcomes.