Recurrent aphthous stomatitis (RAS; aphthae; canker sores) is a common condition which is characterized by multiple recurrent small, round or ovoid ulcers with circumscribed margins, erythematous haloes, and yellow or grey floors typically presenting first in childhood or adolescence. RAS occurs worldwide although it appears most common in the developed world. The aetiology of RAS is not entirely clear. Despite many studies trying to identify a causal microorganism, RAS does not appear to be infectious. A genetic predisposition is present, as shown by strong associations with genotypes of IL-1beta; IL-6 in RAS patients, and a positive family history in about one-third of patients with RAS. Haematinic deficiency is found in up to 20% of patients. Cessation of smoking may precipitate or exacerbate RAS in some cases. Ulcers similar to RAS may be seen in human immunodeficiency virus disease and some other immune defects, and drugs, especially non-steroidal anti-inflammatory drugs and nicorandil may produce lesions clinically similar to RAS. Topical corticosteroids can often control RAS. However, the treatment of RAS remains unsatisfactory, as most therapies only reduce the severity of the ulceration and do not stop recurrence.