More than 90% of malignant tumors arising in humans are of epithelial origin. The loss of epithelial morphology and the acquisition of mesenchymal characteristics are important early events in tumor progression. This type of morphological transformation is termed an epithelial-mesenchymal transition. These transitions occur normally during embryonic development, as well as pathologically during tumor progression. This review will encapsulate our understanding of the role that epithelial-mesenchymal transitions play during tumor progression and metastasis, and we also summarize recent results describing the roles played by such genes as E-cadherin, Snail, and TGFβ in regulating epithelial-mesenchymal transitions and metastasis.