Ulcerations are common occurrences in the mouth. Causes include physical trauma, radiation, chemical injury, and microbial infection (bacterial, viral, and fungal). Some ulcerations, such as recurrent aphthous stomatitis, Behçet's syndrome, and erythema multiforme, are of uncertain etiology, whereas others (eg. pemphigus, pemphigoid) are apparently of immunologic origin. Malignant neoplasms also may present as ulcerations. Because the natural history and treatment varies with the diagnosis, the practitioner should become familiar with the clinical appearance of the various types of ulcerations so that appropriate treatment can be instituted.