Purpose: Dental implants generally provide good results as replacements for missing teeth, but a few patients experience implant failure. The aim of this retrospective study was to analyze the characteristics and causes of implant failures in hopes of reducing future failures. Materials and Methods: Patients who received one or more implants at the Dental Hospital of Yonsei University College of Dentistry between February 1991 and May 2009 were included in this study. Data including age, sex, medical history, habits (eg, smoking and drinking), bone quality, primary stability, implant size, implant surface, additional surgical procedures, prosthetic type, clinical symptoms, implant failure date, and causes of implant failure were obtained through a chart review. Follow-up radiographs were compared to those obtained at baseline. The Pearson chi-square test and Fisher exact test were used to evaluate the correlations between risk factors and implant failure. Results: In total, 879 patients received 2,796 implants; 150 implants in 91 patients had failed. Early and late implant failures occurred with 86 (57.3%) and 64 (42.7%) implants, respectively. The main causes of early and late implant failures were inflammation (47%) and overloading (53%), respectively. When the cause of early implant failure was inflammation, the failure rate was significantly higher for implants in the anterior maxilla; implants with poor primary stability, a machined surface, or a length exceeding 15 mm; and implants placed with a reconstructive procedure and two-stage surgery. When late implant failure was caused by overloading, the failure rate was significantly higher for implants with a machined surface, placed with a reconstructive procedure and/or two-stage surgery, and supporting telescopic dentures. Conclusion: Within the limitations of this study, the major causes of implant failure are inflammation and overloading, and they differ between early and late implant failures. Keywords: causes of implant failure, early implant failure, inflammation, late implant failure, overloading