Ulcers and/or erosions can be the final common manifestation, often clinically indistinguishable, of a wide and complex spectrum of conditions including traumatic lesions, infectious, vesiculo-bullous, neoplastic and gastrointestinal diseases. Their formation represents the final outcome of a complex and finely orchestrated phenomenon involving both epithelial lining and chorion. Reduction of blood flow, cytokine production, cell death, wound repair, all participate to the dynamic process which we define as ''ulcer''. However, little is known about the mechanisms which crucially contribute in determining the phenomenology of ulcer, and some questions still remain unsolved. This paper aims to explain the pathophysiology of oral ulcers attempting to answer three questions: the mechanism of ulcer development, the site of appearance, and the factors which determine the severity and healing time of ulcerative lesions.