TGF-b is a ubiquitous cytokine that plays an active role in many cellular processes. Nearly every cell type has the ability to secrete TGF-b, as well as the ability to respond to TGF-b via the presence of TGF-b receptors on the cell surface. Consequently, gain or loss of function of the TGF-b pathway and its components are known to lead to a variety of diseases, including cancer. In epithelial cells, TGF-b functions as a tumor suppressor, where it inhibits proliferation, induces apoptosis, and mediates differentiation. Conversely, in other contexts, TGF-b promotes tumor progression through increasing tumor cell invasion and metastasis. Thus, TGF-b can have opposing roles, likely dependent, in part, on whether the cancer is early or late stage. The effects of TGF-b on tumor suppression and promotion are not limited to the tumor cell itself; rather, these effects can also be mediated through the stroma and the immune system. The dichotomous role of TGF-b in cancer highlights our need to understand the contextual effects of this cytokine to better guide patient selection for the use of anti-TGF-b therapies currently in clinical trials.