Recurrent aphthous stomatits (RAS) is also known as recurrent oral ulcers, recurrent aphthous ulcers, or simple or complex aphthosis. RAS is the most common inflammatory ulcerative condition of the oral mucosa in North American patients. RAS has been the subject of active investigation along multiple lines of research including epidemiology, immunology, clinical correlations and therapy. Clinical evaluation of the patient requires correct diagnosis of RAS and classification of the disease based on morphology (MIAU, MJAU, HU) and severity (simple versus complex). In order to properly diagnose and treat a patient with lesions of RAS, the clinician must exclude other causes of acute oral ulcers. Complex aphthosis and complex aphthosis variants associated with systemic disorders should be considered. The aphthous-like oral ulcerations of patients with HIV disease represent a challenging differential diagnosis. The association of lesions of RAS with hematinic deficiencies and gastrointestinal diseases provides an opportunity to identify a "correctable cause" which, with appropriate treatment, can result in a remission or substantial lessening of disease activity. Finally, when all of these factors are considered, the evaluation of the patient for BehÇet's disease can be continued on firm grounds that one of the major criteria for the diagnosis of BehÇet's disease has been met.