Five years have passed since the 2018 classification of periodontal and peri-implant diseases were introduced, and we have had a chance to use it in teaching, research, and practice. We have had an opportunity to identify grey areas, as well as to appreciate its utility in identifying patients with disease and exchanging information between colleagues. Drs. Papapanou, Kumar, and Thacker have put together an integrated presentation that will explore the rationale behind the development of the current classification, the biological aspects of disease based on the staging and grading, and clinical scenarios for decision making during patient care and teaching dental students and colleagues.