Transforming growth factor β (TGF-β) plays an important role in tumor initiation and progression, functioning as both a suppressor and a promoter. The mechanisms underlying this dual role of TGF-β remain unclear. TGF-β exerts systemic immune suppression and inhibits host immunosurveillance. Neutralizing TGF-β enhances CD8+ T-cell- and NK-cell-mediated anti-tumor immune responses. It also increases neutrophil-attracting chemokines resulting in recruitment and activation of neutrophils with an antitumor phenotype. In addition to its systemic effects, TGF-β regulates infiltration of inflammatory/immune cells and cancer-associated fibroblasts in the tumor microenvironment causing direct changes in tumor cells. Understanding TGF-β regulation at the interface of tumor and host immunity should provide insights into developing effective TGF-β antagonists and biomarkers for patient selection and efficacy of TGF-β antagonist treatment.