The advent of plastic materials permitting well tolerated contact with human tissue was followed by the appearance of IUDs of various forms on the market. The number of users worldwide has continued to grow and was estimated at 55 millionn in 1982. The form of the IUD should offer protection against pregnancy, permit easy insertion, and remain in place until the patient decides to remove it. Few IUDs meet all 3 criteria. Small devices are easily inserted but more likely to be expelled, while large devices are less likely to be expelled but more likely because bleeding. 1st generation IUDs were inert, while 2nd generation bioactive IUDs liberate copper or progestagens. In Belgium, 8 types of IUDs are available, of which only the Lippes Loop is not bioactive. The bioactive devices are primarily copper IUDs. The duation of protection is 3 years except for the Nova T which appears to give protection for 4 years. The mode of action of IUDs has been the subject of controversy. As a foreign object the IUD appears to produce continuous small contractions which are sometimes painful. The receptivity of the endometrial mucus is decreased by a leukocytic reaction, which discourages implantation of the fertilized ovum. The contractions probably extend to the tubes, interrupting the usual process of fertilization. The contraceptive effect is undoubtedly due to more than 1 mechanism, and the bioactive follicular phase because of the cervical hypotony at this stage, but insertion during menstruation, after pregnancy of abortion, or after unprotected intercourse may be advantageous in some cases. A preliminary examination should rule out uterine fibroma, ovarian cysts, and cervical or adnexial infection. About 10% of users experience side effects including bleeding, pain, vaginal discharge, infection, or perforation. Observations over several years have demonstrated that longterm IUD use does not have serious negative consequences. The expulsion rate is 10% but 1/2 occur in the 1st 3 months of use and expulsions after the 1st year are rare. The pregnancy rate is about 2% for the 1st year use. Pregnancy can continue with the device in place, but a spontaneous abortion or ectopic pregnancy may occur. IUD contraception has the advantages of simplicity and ease of use.