Assessment of endodontic treatment outcomes has evolved from disease-based criteria proposed by Strindberg to patient-centred values, emphasising survival and function of endodontically treated teeth even in the presence of inflammatory peri-apical disease. It is essential that patients are fully informed of the distinction between disease-free and disease-associated treatment outcomes. Ideally, disease-free treatment outcomes should always be the goal of all endodontic treatments.Treatment success can be based on a variety of factors including but not limited to symptom resolution, the healing of peri-apical tissues per radiograph, or even histologic evidence showing the absence of infected cells. Patients, on the other hand, may define success differently than practitioners. A patient's 'definition' of success may be the absence of pain or increased functionality of the tooth. Patients want treatment options that are proven to produce results in a timely and feasible manner that will last. The use of the PAI for evaluating treatment results has been advised because it is a reliable and valid way for denoting a healthy peri-apex and hence, scientific evidence of 'success'.