Peri-implantitis is an inflammatory disease that occurs around dental implants and damages both soft and hard tissues, the characteristic feature of which is bone loss. The major etiology of peri-implantitis is dental plaque, including implant overload implants, a history of periodontitis, smoking and diabetes as risk factors. The standards for the clinical diagnosis of peri-implantitis are bleeding on probing, suppuration, a peri-implant pocket depth ≥5 mm, and X-ray evidence. Treatment includes mechanical debridement, drug therapy, laser treatment and surgical treatment. Regular supportive peri-implant therapy can be effective for curing and preventing peri-implantitis. In this paper, the etiology, clinical examination and treatment of periodontitis are reviewed.