Carcinosarcomas are rare, biphasic tumors that are comprised of carcinomatous and sarcomatous elements. While the exact mechanism by which these two phenotypes arise within a single tumor remains unclear, molecular evidence indicates that the epitheliod and spindle-cell components share a clonal origin. We propose that the biphasic nature of these neoplasms may represent an extreme case of epithelial plasticity, in which an epithelial-like cell undergoes a transition to a more mesenchymal phenotype. The present review will discuss both the histological and molecular biological evidence of the involvement of epithelial plasticity in driving the mixed phenotypes observed in carcinosarcomas.