The literature concerning the success rates and the causes of failure of dental implants is reviewed in this paper, and the factors influencing clinical outcome are discussed. Most dental implants seem to be clinically acceptable during the first few years of installation, as long as the initial healing is uncomplicated. On a long-term basis, however, osseointegrated implants are the most reliable among those currently available for routine use. The lack of sufficient scientific data and well-controlled clinical studies with adequate criteria for assessing successful outcome are the main reasons why most other dental implants cannot be clinically recommended without criticism.