From a public health viewpoint, the intrauterine device (IUD) is the most widely used contraceptive method in the world. Prevalence rates range among countries from 2 to 80% of contraceptive users. During 5 years of IUD use, pregnancy occurs in less than 2 per 100 insertions. Bleeding and pain are the most common reasons for removal rates of 10% in the first year and up to 50% within 5 years. The contraceptive effects of IUDs may be due to a sterile inflammatory reaction in the endometrial cavity which interferes with sperm function, so that fertilization is less likely to occur. IUDs also interfere with implantation but the extent to which this contributes to their contraceptive action is unknown. In nulliparous women rates of expulsion and removal for bleeding and/or pain are higher than in parous women. Effective use of IUDs for up to 10 years has the same pregnancy rate as tubal interruption. Thus, the IUD may be an alternative to female sterilization, especially in younger women who are more likely to experience regret after sterilization. The levonorgestrel intrauterine system (LNG-IUS) reduces bleeding and dysmenorrhoea, provides superior effectiveness to copper IUDs, and may be a useful treatment for endometriosis or an alternative to hysterectomy for menorrhagia.