‘Apical periodontitis’ is a general term used to describe the periapical inflammatory process that occurs in response to the presence of micro-organisms and other irritants within the root canal system of a tooth. Although many patients will develop apical periodontitis without having symptoms for a long period of time, it is very likely that there will be an acute exacerbation at some stage and then various signs and/or symptoms will become obvious. However, there are other conditions that can mimic apical periodontitis – such as an ‘extension’ of pulpitis, periodontal disease, occlusal trauma, an accident that has damaged the periodontal ligament, and various tumors or cysts. Hence, it is essential that dental practitioners understand the progressive nature of the periapical disease process as well as how and why the various stages occur so they can be diagnosed and managed appropriately. The diagnosis will usually be based on the clinical and radiographic manifestations and the results of the various tests that can be performed as part of a routine dental examination.