The classification from 2017 defines two major groups: periodontal diseases and conditions and peri-implant diseases and conditions. Periodontal diseases and conditions are then divided into three groups: periodontal health and gingival diseases and conditions, periodontitis, and other conditions of the periodontium. Periodontal health is defined by absence of clinically detectable inflammation. Clinical health of the gingiva can be present on an intact or reduced periodontium and can be re-established after the treatment of gingivitis and periodontitis. Gingiva conditions can be divided into two major categories: gingiva diseases caused by plaque and those which are not caused by plaque. Plaque-induced gingivitis can be present on an intact or reduced periodontium, both in patients who are periodontitis-free and those whose periodontitis has been treated successfully. The new classification puts the former 'chronic' and 'aggressive' periodontitis into a common category called 'periodontitis' and characterizes them on the basis of a stage and class system. The stage system tells us about the severity of the disease and complexity of its treatment, whereas the class system describes the biological features of the periodontitis, the risk of further progression and analysis of possible negative treatment outcomes. The new classification divides periodontitis into necrotizing periodontal diseases, periodontitis and periodontitis as a manifestation of systemic diseases. Many systemic diseases and conditions can have a negative impact on the periodontium or influence the course of periodontitis. Gingiva recessions can be linked to oversensitivity, carious development and non-carious cervical lesions on the exposed surface of the root. Occlusal forces may cause teeth or periodontal support injuries. Some developed or acquired conditions related to teeth or prosthetic replacements can cause periodontal illness. Endodontic-periodontal diseases and periodontal abscesses are also included in conditions which affect periodontium. The classification of peri-implant diseases and conditions is presented and it describes the features of peri-implant health, peri-implant mucositis, peri-implantitis and soft and hard tissue deficiencies. Peri-implant health is defined by absence of erythema, probing bleeding, swelling and suppuration, and can occur in implants with reduced bone support. The characteristic of peri-implant mucositis is bleeding on gentle probing, the main etiological cause being plaque. Peri-implantitis is a pathological condition in the tissue around dental implants due to plaque and characterized by inflammed peri-implant mucosa with the consequence of supporting bone loss.