This narrative review was prepared for the 2017 World Workshop of the American Academy of Periodontology and European Federation of Periodontology to address key questions related to the clinical condition of peri-implant mucositis, including: 1) the definition of peri-implant mucositis, 2) conversion of peri-implant health to the biofilm-induced peri-implant mucositis lesion, 3) reversibility of peri-implant mucositis, 4) the long-standing peri-implant mucositis lesion, 5) similarities and differences between peri-implant mucositis at implants and gingivitis at teeth, and 6) risk indicators/factors for peri-implant mucositis.