Activation of the transcription factor NF-?B occurs in many human tumors, and studies have shown that NF-κB can promote cell proliferation and oncogenesis, possibly by protecting cells from apoptosis. Little is known, however, about whether NF-κB is involved in tumor progression including epithelial-mesenchymal transition (EMT), a central process governing both morphogenesis and carcinoma progression in multicellular organisms. In a combined in vitro/in vivo model of mammary carcinogenesis, NF-κB was essential both for the induction and maintenance of EMT and for in vivo metastasis. NF-κB and the signaling pathways that are involved in its activation should therefore receive attention as potential targets for the development of novel anti-metastatic cancer treatments.