Purpose: The aim of this narrative review was to explore and discuss marginal bone loss around transmucosal oral implants and the related incidence of this biologic complication. Results: Treatment with osseointegrated implants is most often successful and improves the quality of life for the patient. At present only limited data are available to evaluate long-term technical and biological complications. When pen-implant tissue destruction occurs, little is known about the initiating process. Possible factors of relevance for the initiation and progression of peri-implantitis are discussed. Periodontitis, smoking and a variety of local factors are among the most plausible putative reported risk factors. Also, oral hygiene and the inability to clean the reconstruction were reported. The unit for reporting incidence of pen-implant bone loss varies in different studies between implants and patients. Since there seems to be a clustering effect, and implants in the same mouth cannot be considered independent from each other, it is recommended to use the patient as a unit. The different cut-off values for clinical parameters reported in different studies will exert a significant influence on the magnitude of the reported incidence of peri-implantitis. It is suggested that the composite variables including bone loss >= 2 mm, compared to initial radiographs at delivery of the prosthetic device, in combination with bleeding on probing should be interpreted as a 'red flag' for the clinician to critically evaluate if any intervention is indicated in the individual case. Conclusion: Until more solid scientific evidence has been made available, it is likely that the academic controversy in relation to pen-implant bone loss and peri-implantitis will continue.