Recurrent aphthae (or aphthoses) are the most frequent inflammatory lesions of the oral mucosa. Aphthae can clearly be defined, but may also be barely discernible from aphthoid erosions and ulcers of heterogeneous origin. Although a subject of molecular research, the etiopathogenesis of oral aphthoses is still unknown. However, the clinical differentiation of a "benign" type of aphthosis from a "pernicious" one, such as Behçet's disease (BD), is a diagnostic matter of great significance. BD has been found clinically in increasing numbers in central Europe and represents an (auto-)hyperimmune multisystem life-threatening vasculitis. In this disorder possible damage can occur to many visceral organs and/or the cerebrospinal system. The prognosis of BD may depend on the changing involvement of very different organs, as well as the early recognition of the disease per se. The most prominent feature is a multilesional aphthous stomatitis that is almost never absent during acute episodes of BD. Hence, the occurrence of this peculiar type of aphthosis strongly indicates an active BD. The plethora of other aphthoid lesions can be challenging to the diagnostic competence of all physicians and may confuse the correct recognition of the severity of the disease present. This review particularly details the wide range of diagnostic, therapeutic and prognostic aspects of the various oral aphthoses.