Transforming growth factor beta (TGFβ) is a multifunctional polypeptide. Its role in carcinogenesis can be either suppressive or promoting depending on tumor developmental stages and cellular context. During the early phase of epithelial tumorigenesis, TGFβ inhibits primary tumor development and growth by inducing cell cycle arrest and possibly apoptosis. However, in late stages of progression, as tumor cells evade the growth inhibition by TGFβ due to inactivation of its signaling pathway or aberrant regulation of cell cycle machinery, the role of TGFβ signaling is often switched from tumor suppression to promotion. TGFβ can apparently act in tumor stroma as well as tumor cells to inhibit host immune surveillance and stimulate invasion, angiogenesis, and metastasis. Studies have shown that antagonizing TGFβ activity can inhibit tumor progression, especially metastasis, in certain tumor models. However, the molecular markers that can indicate the feasibility of the use of TGFβ antagonists as cancer therapeutics remain to be determined.