Due to increased use of dental implants, various problems related to dental implants are reported. Patients suffering from peri-implant disease are especially increasing. Peri-implant diseases can be classified into peri-implant mucositis which is the inflammation of mucosa without loss of supporting bone and peri-implantitis which is the inflammation of mucosa including loss of supporting bone. Major factor of peri-implant diseases is bacterial infection, and the bacteria is same as those which cause periodontitis of natural teeth (Prevotella intermedia, Porphyromonas gingivalis, Fusobacterium spp.). These peri-implant diseases are diagnosed by clinical examination (gingival condition, bleeding on probing, pocket depth, suppuration, mobility) and radiographic images. Treatment plans are established according to the diagnosis. Cumulative Interceptive Supportive Therapy (CIST), suggested by Mombelli et al., is a cumulative and gradual therapy and application of CIST makes effective treatment possible. The degree of peri-implant diseases is classified by comparison of pus discharge, bleeding on probing, pocket depth and bone loss on radiographic images and according to this classification, treatment is planned for gradual application of mechanical cleansing, disinfectant therapy, antibiotics therapy and surgical treatments. (JOURNAL OF DENTAL IMPLANT RESEARCH 2015;34(3):76-80)