After reviewing the literature on intrauterine devices (IUDs) published over the last 30 years, we selected and analyzed recent and most relevant (with appropriate methodology design) articles dealing with the risk factors for IUD failure. There is sufficient evidence to conclude that the TCu380 and levonorgestrel-releasing (LNG) IUD are the most effective, with a cumulative pregnancy rate of less than 2% at 12 years for the TCu380 and around 1% for the LNG IUD at 7 years. In addition, advancing age increases IUD effectiveness, and displacement of the IUD decreases effectiveness. The IUDs with the highest copper concentrations are clearly less prone to failure than those with lower copper concentrations.