The epithelial-mesenchymal transition is a highly conserved cellular program that allows polarized, immotile epithelial cells to convert to motile mesenchymal cells. This important process was initially recognized during several critical stages of embryonic development and has more recently been implicated in promoting carcinoma invasion and metastasis. In this review, we summarize and compare major signaling pathways that regulate the epithelial-mesenchymal transitions during both development and tumor metastasis. Studies in both fields are critical for our molecular understanding of cell migration and morphogenesis.