The development of malignancy from a pleomorphic adenoma is well recognised but is considerably more complex than previously thought. Apart from tumours with obvious histological malignancy, there are some pleomorphic adenomas that have atypical histological features only and yet others with malignant biological behaviour but with benign histology, the so-called metastasising pleomorphic adenomas (see Chap. 3). Carcinomas ex pleomorphic adenoma are also subdivided into intracapsular (non-invasive, in situ) carcinoma, minimal invasive carcinoma and invasive carcinoma. Poorly differentiated adenocarcinoma not otherwise specified, salivary duct carcinoma and adenoid cystic carcinoma are the most commonly reported histological types of malignancy. Virtually any salivary type carcinoma may develop; however, with the possible exception of acinic cell carcinoma. Usually only one histological type is encountered. On very rare occasions, carcinosarcoma or non-salivary tumours such as malignant rhabdoid tumours may arise. In his extensive review of the literature, Gnepp culled 42 cases of carcinosarcoma, and in at least 11 of these cases there was a previous history of pleomorphic adenoma.