The success of dental implants is highly dependent on integration between the implant and intraoral hard/soft tissue. Initial breakdown of the implant-tissue interface generally begins at the crestal region in successfully osseointegrated endosteal implants, regardless of surgical approaches (submerged or nonsubmerged). Early crestal bone loss is often observed after the first year of function, followed by minimal bone loss (< or =0.2 mm) annually thereafter. Six plausible etiologic factors are hypothesized, including surgical trauma, occlusal overload, peri-implantitis, microgap, biologic width, and implant crest module. It is the purpose of this article to review and discuss each factor Based upon currently available literature, the reformation of biologic width around dental implants, microgap if placed at or below the bone crest, occlusal overload, and implant crest module may be the most likely causes of early implant bone loss. Furthermore, it is important to note that other contributing factors, such as surgical trauma and penimplantitis, may also play a role in the process of early implant bone loss. Future randomized, well-controlled clinical trials comparing the effect of each plausible factor are needed to clarify the causes of early implant bone loss.