Background: To develop preventive strategies addressing peri-implant diseases, a thorough understanding of the epidemiology is required. Aim: The aim was to systematically assess the scientific literature in order to evaluate the prevalence, extent and severity of peri-implant diseases. Material & methods: Data were extracted from identified studies. Meta-analyses for prevalence of peri-implant mucositis and peri-implantitis were performed. The effect of function time and disease definition on the prevalence of peri-implantitis was evaluated by meta-regression analyses. Data on extent and severity of peri-implant diseases were estimated if not directly reported. Results: Fifteen articles describing 11 studies were included. Case definitions for mucositis and peri-implantitis varied. The prevalence of peri-implant mucositis and peri-implantitis ranged from 19 to 65% and from 1 to 47%, respectively. Meta-analyses estimated weighted mean prevalences of peri-implant mucositis and peri-implantitis of 43% (CI: 32-54%) and 22% (CI: 14-30%), respectively. The meta-regression showed a positive relationship between prevalence of peri-implantitis and function time and a negative relationship between prevalence of peri-implantitis and threshold for bone loss. Extent and severity of peri-implant diseases were rarely reported. Conclusion: Future studies on the epidemiology of peri-implant diseases should consider (i) applying consistent case definitions and (ii) assessing random patient samples of adequate size and function time.