When celebrating 100 yr of dental research in the Nordic dental research commu- nity (i.e. Nordisk Odontologisk Fo€rening (NOF)), it is relevant to include dental implant treatment. In essence, the successful progress of implant treatment has added both to the quality of life for patients and also to many aspects of profes- sional development and job satisfaction for dentists. When appreciating the success story it also seems relevant to highlight some of the problems related to this treat- ment. Both technical and biological complications have often been ignored when reporting long-term results following implant treatment. Different opinions have been expressed in relation to the etiology of peri-implant diseases. Some even choose to ignore this condition as a clinical problem. This article presents a short overview of peri-implant diseases (i.e. peri-implant mucositis and peri-implantitis). The lack of internationally agreed disease definitions for peri-implant diseases, as with periodontitis, results in wide variation of estimates for the occurrence of peri- implant diseases when epidemiological data are reported. The profession still strives to find and define the best way to deal with peri-implant diseases once they are accurately diagnosed. Awareness of the tissue conditions in the peri-implant area, and relevant action when indicated, seems to be critical for the continued long-term successful outcome of dental implant treatment.