The TGF-beta signaling pathway controls multiple functions of cancer cells and the surrounding stromal tissue. Some TGF-beta actions suppress cancer formation, while others contribute to tumor progression. Evidence supporting a tumor suppressive role for the TGF-beta/Smad signaling axis is presented here. These data are compiled from cell culture studies, animal models, analyses of human tumors, and investigations of polymorphisms of TGF-beta pathway components and their associated cancer risk. Therapeutic strategies for cancer treatment involving either restoring or potentiating TGF-beta tumor suppressive activities, or blocking TGF-beta tumor promoting functions are considered.