The transforming growth factor-ß (TGF-β) signaling pathway plays a pivotal role in diverse cellular processes. TGF-β switches its role from a tumor suppressor in normal or dysplastic cells to a tumor promoter in advanced cancers. It is widely believed that the Smad-dependent pathway is involved in TGF-β tumor-suppressive functions, whereas activation of Smad-independent pathways, coupled with the loss of tumor-suppressor functions of TGF-β, is important for its pro-oncogenic functions. TGF-β signaling has been considered a useful therapeutic target. The discovery of oncogenic actions of TGF-β has generated a great deal of enthusiasm for developing TGF-β signaling inhibitors for the treatment of cancer. The challenge is to identify the group of patients where targeted tumors are not only refractory to TGF-β-induced tumor suppressor functions but also responsive to the tumor-promoting effects of TGF-β. TGF-β pathway inhibitors, including small and large molecules, have now entered clinical trials. Preclinical studies with these inhibitors have shown promise in a variety of different tumor models. Here, we focus on the mechanisms of signaling and specific targets of the TGF-β pathway that are critical effectors of tumor progression and invasion. This report also examines the therapeutic intervention of TGF-ß signaling in human cancers. © 2010 Informa UK Ltd.