Diagnosis is the identification of the nature of an illness or other problem by examination of the symptoms. The physician formulates a hypothesis of most likely provisional diagnosis and in many cases will obtain further investigations to confirm or clarify the diagnosis before providing treatment. Once a diagnosis has been reached, the physician is able to propose a management plan which will include treatment as well as plan for follow up. A proper and true periodontal diagnosis is essential in order to have a rational treatment and preventive strategy and to identify sites at risk in periodontitis patients. The term diagnostic criteria designate combination of signs, symptoms and the test results that allows the physician to ascertain the diagnosis of disease.