Malignant tumors with a mixed phenotype are a controversial field of pathology. In this article the morphological aspects and the immunohistological characterization of sarcomatoid carcinomas are presented. These uncommon neoplasms show both carcinomatous and sarcomatous features, and have been described in the past under a variety of different names causing great uncertainty about their classification and histogenesis. They can occur in various anatomical sites and exhibit a wide range of microscopic appearances, but some features are quite characteristic and are found in many cases. Morphological "transition" between carcinomatous and sarcomatous tissue, and detection of epithelial characteristics by electron microscopy or immunohistochemistry in the sarcomatous component, are very peculiar features of these neoplasms, providing both helpful clues for pathological diagnosis and important insights into histogenesis. Here a unifying histopathogenetic mechanism based on the phenotypic conversion of carcinoma into sarcomatoid tissue is proposed and supporting literature data from both experimental systems and clinicopathological observations are reviewed and discussed.