Osseointegrated dental implants are used routinely in dentistry in the confidence of predictable success. However, if the implant surfaces become colonised by pathogenic bacteria, the plaque-induced inflammation around the implants may cause peri-implant tissue destruction. Peri-implant mucositis is a reversible, plaque-induced inflammatory lesion confined to the peri-implant soft tissue unit and clinically is characterised by redness, swelling and bleeding on gentle probing. Peri-implantitis is an extension of peri-implant mucositis to involve the bone supporting the implant: it is characterised by loss of osseointegration of the coronal part of the implant, by increased probing depth and by bleeding and/or suppuration on probing. Established peri-implantitis does not respond predictably to treatment. The best management of plaque-induced peri-implant inflammatory diseases is prevention. Regular personal and professional cleaning of the implant is mandatory to minimise bacterial load. Despite our best efforts, plaque-induced peri-implant inflammatory diseases will occur frequently, and as these diseases respond best to early treatment, early detection of peri-implant mucositis by regular assessment will permit timely treatment. Peri-implant mucositis is readily treated non-surgically. Peri-implantitis is more difficult to treat largely because of the problem of decontamination of the roughened, threaded surfaces of exposed implants. As a rule, surgical treatment will be necessary, and even then success is not assured.