Purpose: This article provides a critical examination of the literature on the colonization of the peri-implant environment and its impact on clinically significant infection, in the context of a comparison with the microbial etiology of periodontitis. Materials and Methods: Previous journal article bibliographies and MEDLINE searches provided comprehensive access to current literature on this topic. References that provided significant or novel advancements in knowledge were included in this review. Since it was prepared for a workshop that would examine this issue critically, this review offers provocative commentary. Results: There are many similarities in peri-implant and periodontal microbiology, both in health and in infection leading to progressive deterioration. Periodontal pockets serve as reservoirs for the colonization of implants by periodontal pathogens in partially edentulous patients. Implant design and material composition affect the colonization of implant surfaces. Infections around implants can be treated by similar strategies used for periodontitis. Investigators are beginning to adopt research designs used in periodontology to study the pathogenesis and treatment of peri-implant infections. Research in this area is impeded by the lack of surrogate variables for crestal bone resorption. Conclusion: Some implants fail as a result of infection, but the frequency of failure by infection or any other cause is unknown. This paper suggests areas where much more additional research is needed, including critical consideration of strategies that have led to the past few decades' advancements in the pathogenesis of periodontitis.