This narrative review addresses conventional diagnostic criteria used in clinical practice to discriminate between periodontal health, gingivitis, and periodontitis. Visual examination of the color and texture of the periodontal tissues, assessment of plaque deposits, periodontal probing assessments, and diagnostic imaging enable the collation of information to make a periodontal diagnosis, followed by an appropriate treatment plan. The periodontal probe is an essential diagnostic tool to assess probing pocket depth, clinical attachment level, bleeding on probing, and the degree of furcation involvement at multirooted teeth. When clinical signs and symptoms of periodontitis are identified, diagnostic imaging enables evaluation of the level and extent of bone destruction and bone defect morphology. The diagnostic process requires clinicians who are trained to evaluate, record, and interpret these measures. This narrative review focuses on conventional clinical diagnostic parameters which, despite their limitations, are considered the current standard of care.