**1. Introduction**

Cancer metastasis is the major cause of cancer morbidity and mortality. This process ac‐ counts for approximately 90% of cancer deaths. Epithelial-mesenchymal transition (EMT) is a characteristic of the majority of metastatic cells. EMT is a natural transdifferentiation mecha‐ nism that governs changes in cell states along the epithelial versus mesenchymal axes and confers epithelial-mesenchymal plasticity upon epithelial cells. In particular, epithelial cells are transformed from highly differentiated, polarized, and organized cells into undifferenti‐ ated, isolated, and mesenchymal-like cells with migratory and invasive properties. In this chapter, we summarize evidence supporting the widespread involvement of EMT in tumor pathogenesis and the regulation of cancer metastasis.

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