The article reviews the importance of epithelial-mesenchymal transition (EMT) in embryonic development and disease. EMT is a process where cells change from an epithelial to a mesenchymal state, allowing them to migrate and invade tissues. The review highlights two key developmental events governed by EMT: gastrulation and neural crest formation. During gastrulation, cells in the embryo layer called the epiblast undergo EMT to form the three embryonic layers (ectoderm, mesoderm, and endoderm). The neural crest, a unique evolutionary feature, arises from the neural tube and migrates to form various structures in the peripheral nervous system and other tissues. The article also discusses how defects in EMT can lead to congenital malformations and how reactivation of EMT programs in adults can contribute to diseases such as organ fibrosis and tumor progression. Understanding EMT during embryonic development may provide insights into designing drugs to prevent or treat these diseases.The article reviews the importance of epithelial-mesenchymal transition (EMT) in embryonic development and disease. EMT is a process where cells change from an epithelial to a mesenchymal state, allowing them to migrate and invade tissues. The review highlights two key developmental events governed by EMT: gastrulation and neural crest formation. During gastrulation, cells in the embryo layer called the epiblast undergo EMT to form the three embryonic layers (ectoderm, mesoderm, and endoderm). The neural crest, a unique evolutionary feature, arises from the neural tube and migrates to form various structures in the peripheral nervous system and other tissues. The article also discusses how defects in EMT can lead to congenital malformations and how reactivation of EMT programs in adults can contribute to diseases such as organ fibrosis and tumor progression. Understanding EMT during embryonic development may provide insights into designing drugs to prevent or treat these diseases.