The aim of this review was to determine the most common peri-implant mucositis and peri-implantitis case definitions used worldwide in the implant dentistry literature. A systematic assessment of peri-implant disease classification was conducted using all publications in MEDLINE, EMBASE, SCOPUS, and Google Scholar between 1994 and November 2017. Screening of eligible studies and data extraction were conducted in duplicate and independently by 2 reviewers. The search protocol identified 3049 unique articles, of which 2784 were excluded based on title and abstract. In total, 265 full texts were screened, 106 of which met the eligibility criteria. Of these, 41 defined peri-implant mucositis. Eight (19.6%) used bleeding on probing (BOP) only; 8 (19.6.7%) used a combination of probing depth (PD), BOP, and radiograph; and 5 (12.3%) used PD and BOP. Cases with crestal bone loss of ≤2 mm in the first year and ≤0.2 mm in each subsequent year were considered as peri-implant mucositis. Ninety-three articles defined peri-implantitis; 28 (30.1%) used a combination of PD with suppuration, BOP, and radiograph, followed by 25 (26.9%) using a combination of PD, BOP, and radiograph. The main criteria in most of the studies were considered to be BOP, PD, and radiograph. Cases of crestal bone loss of ≥2 mm and PD ≥3 mm are considered peri-implantitis. Different peri-implant disease case definitions may affect disease prevalence and treatment strategies. We need to standardize case definitions to avoid discrepancies in case diagnosis and prognosis.