The goals of this review are as follows: (1) To update the markers associated with the epithelial and mesenchymal phenotype, (2) to describe the inducers associated with EMT and tumor progression, (3) to discuss the signaling pathways induced by leptin, and (4) to report the most recent findings from in vitro, in vivo, and patient studies of the role of leptin as an inducer of EMT in cancer. The gene discussed is LEP; the disease is cancer.