Oral ulcers represent a comprehensive group of lesions. Because they are both common and painful, the dentist and physician are frequently consulted for diagnosis and treatment. The pathogenesis, clinical appearance, and suggested treatment modalities for the most common ulcerations are presented. Knowledge of the classification of oral ulcers will aid in proper diagnosis and direct the clinician toward appropriate therapy. In many cases, only palliative care can be offered, but symptoms can often be ameliorated.