Abstract: Chemotherapy, molecular targeted therapy, and hormonal therapy are essential components of medical oncology. Although cancer patients significantly benefit from the emergence of various new anticancer drugs, none of these treatments can directly address drug resistance. Radiation therapy is one of the three conventional cancer treatment methods. Nearly two-thirds of cancer patients accept radiation therapy during treatment. However, radiation resistance is a significant barrier affecting the therapeutic effect of this procedure. Epithelial–mesenchymal transition (EMT) is a biologic process that enables a polarized epithelial cell to undergo multiple biochemical changes. These changes enable the cell to assume the functions of a mesenchymal cell phenotype. These functions have been extensively studied and are related to embryogenesis, tumor invasiveness, and metastasis. In recent years, increasing evidence suggests that EMT is closely linked with tumor treatment resistance. The study of the relation between EMT and tumor treatment resistance is expected to contribute to the prevention of drug resistance and radiation resistance and thus improve treatment efficacy to provide benefit to cancer patients. This article explores this issue.