Transforming growth factor (TGF)-beta is a potent growth inhibitor of various types of cells, whereas it stimulates deposition of extracellular matrix proteins and induction of epithelial-to-mesenchymal transition (EMT). TGF-beta thus plays two distinct and opposing roles in cancer progression. In the present study, we have developed a useful method that enables monitoring of tumor and its TGF-beta signaling within the same animal using in vivo bioluminescent imaging.