Epithelial-mesenchymal transition (EMT) is a key step during embryonic morphogenesis, heart development, chronic degenerative fibrosis, and cancer metastasis. Several distinct traits have been conveyed by EMT, including cell motility, invasiveness, resistance to apoptosis, and some properties of stem cells. Many signal pathways have contributed to the induction of EMT, such as transforming growth factor-β, Wnt, Hedgehog, Notch, and nuclear factor-κB. Over the last few years, increasing evidence has shown that EMT plays an essential role in tumor progression and metastasis. Understanding the molecular mechanism of EMT has a great effect in unraveling the metastatic cascade and may lead to novel interventions for metastatic disease.