Squamous cell carcinoma (SCC) is the most prevalent malignant neoplasm of the oral cavity. The conventional tumour and several histologic subtypes of SCC present morphologic features and specific behaviour when they occur in the oral mucosa. For example, basaloid SCC is an aggressive tumour and verrucous carcinoma has the lowest invasive and metastatic potential; however, the clinical and biologic course of these oral SCC variants has not been completely established. Furthermore, although numerous clinical and histologic features associated with oral SCC have been identified, none shows unequivocal prognostic significance. The purpose of this article is to review the various subtypes of oral SCC, emphasizing problems in their histologic diagnosis and prognosis.