Millions of partially or fully edentulous patients worldwide have been treated by means of dental implants ever since the description of the osseointegration process by PI Brånemark. Despite high success rates, different biological and mechanical complications do occur. Peri-implantitis is a chronical infection around dental implants with irreversible crestal bone loss. Like periodontitis, peri-implantitis is a multifactorial disease caused by pathogenic species in a sensitive host. Today treatment of peri-implantitis is highly unpredictable, hence regular follow-up and prevention seems warranted. As the number of patients rehabilitated with dental implants is growing, the incidence will certainly increase. Guidelines about diagnostics, prevention and treatment of peri-implantitis are mentioned in this article based on current scientific evidence.