Recurrent Aphthous Stomatitis (RAS) is a non-infectious disease of the mucous membrane in the oral cavity characterized by painful ulcers which recur in cycles. Oral ulcers may vary according to their number and size depending on the clinical presentation and can be classified into three categories: small Mikulicz's aphthous ulcers, big Sutton's aphthous ulcers, and herpes-like aphthous ulcers. Some authors distinguish additional forms of RAS: those constituting Behcet Syndrome, and PF APA Syndrome. The lesions to the mucous membrane appear in healthy patients as well as in patients suffering from systemic diseases. Since the etiology of RAS remains unknown there are no effective methods of its prevention or treatment.Recent research shows, however, that disorders in immunological response are present in patients with RAS. Due to pain associated with the condition aphthous stomatitis constitutes a serious therapeutic problem, especially in patients prone to frequent recurrence of the disease. The treatment of patients depends on the severity and type of RAS and cannot be restricted only to its symptoms but also an underlying systemic disease has to be taken into consideration. According to data a universal approach to RAS does not exist, which results from its complex, multifarious etiopathogenesis. The article presents state-of the-art findings concerning the definition, classification, etiology, epidemiology, and methods of diagnosis and treatment of Recurrent Aphthous Stomatitis.