The lineage transition between epithelium and mesenchyme is a process known as epithelial–mesenchymal transition (EMT), by which polarized epithelial cells lose their adhesion property and obtain mesenchymal cell phenotypes. EMT is a biological process that is often involved in embryogenesis and diseases, such as cancer invasion and metastasis. The EMT and the reverse process, mesenchymal–epithelial transition (MET), also play important roles in stem cell differentiation and de‐differentiation (or reprogramming). In this review, we will discuss current research progress of EMT in embryonic development, cellular differentiation and reprogramming, and cancer progression, all of which are representative models for researches of stem cell biology in normal and in diseases. Understanding of EMT and MET may help to identify specific markers to distinguish normal stem cells from cancer stem cells in future. In this review, we discuss current research progress of EMT in embryonic development, cellular differentiation and reprogramming, and cancer progression, all of which are representative models for researches of stem cell biology in normals and in diseases.