A classification system for implant-supported overdentures is proposed in this article. Various types of implant-supported over-dentures have been described in the literature, but there is considerable difference in the function of these designs. For example, an overdenture supported by free-standing implants does not have the retentive elements provided by a bar and, therefore, is less retentive than an overdenture supported by a straight bar. If more than two implants are configured in an arch, and if indirect retention is incorporated into the bar and overdenture design, function is improved over that of a straight-bar design. An-other classification of overdenture is one that receives all of its support from an implant-supported bar with tissue contact to provide a mucosal seal. This tissue con-tact does not provide any actual support. By accepting and using this classification system, the clinician can design an overdenture that is appropriate for the functional requirements of the patient being treated. (Implant Dent 1999; 8:68–73)