Etiology and Epidemiology: The Greek term aphthai was initially used in relation to disorders of the mouth and is credited to Hippocrates (460-370 BC). Today, recurrent aphthous ulceration, or recurrent aphthous stomatitis (RAS), is recognized as the most common oral mucosal disease known to human beings. Considerable research attention has been devoted to elucidating the causes of RAS; local and systemic conditions, and genetic, immunologic, and infectious microbial factors all have been identified as potential etiopathogenic agents (Table 1). However, to date, no principal etiology has been discovered. Epidemiologic studies indicate that the prevalence of RAS is between 2% and 50% in the general population; most estimates fall between 5% and 25%. In selected groups, such as medical and dental students, it has been observed with a frequency as high as 50% to 60%. The peak age of onset for RAS is between 10 and 19 years. After childhood and adolescence, it may continue throughout the entire human lifespan without geographic or age-, sex-, or race-related preference.