The lack of a standardized classification to differentiate the various degrees of peri-implantitis has resulted in confusion when interpreting the results of studies evaluating the prevalence, treatment, and outcomes of therapy. The purpose of this paper is to propose a classification for peri-implantitis based on the severity of the disease. A combination of bleeding on probing and/or suppuration, probing depth, and extent of radiographic bone loss around the implant is used to classify the severity of peri-implantitis into early, moderate, and advanced categories. The rationale and method of measurement for the classification are presented and discussed. This classification should help in communication between researchers and clinicians and thus provide a better understanding of peri-implantitis.