Aphthous stomatitis remains an illness of unknown aetiology, probably drawing on immunological mechanisms. The diagnosis of an oral aphthous ulcer is clinical, corresponding to a precise primary lesion. The benign form is the most frequent, however the illness sometimes evolves into a severe form which can then be considered as a generalised illness. The management of the patient must distinguish the evolving clinical form of the aphthous ulcer, search for a likely aetiology, and employ a therapeutic approach, which actually remains disappointing with the basic treatments being of solely temporary efficacity.