Developing a diagnosis is, in essence, a process in information gathering. This information is gathered from the patient’s history of pain and/or symptoms, trauma or restorative procedures, clinical examinations, results of clinical tests and radiographic examination of the teeth and the surrounding tissues. The diagnostic process begins with a patient interview and review of the medical history, dental history and pain history. A thorough patient and pain assessment interview will frequently enable a clinician to differentiate between odontogenic and non-odontogenic pain. Non-odontogenic pain will often become chronic and debilitating in nature. Furthermore, this pain can be further exacerbated by incorrect or unnecessary treatments at times resulting in the establishment of chronic pain pathways. An accurate diagnosis is crucial for arriving at an appropriate treatment choice: i.e., no treatment, medication, vital pulp therapy, non-surgical root canal treatment, surgical root canal treatment, extraction or referral. Pulpal and periradicular testing should always be carried out to help establish a correct diagnosis. Ultimately the clinician must answer two critical questions before progressing to treatment of a tooth: (1) is the tooth restorable and (2) what is the periodontal prognosis for the tooth.