Transforming growth factor-β (TGFβ) is an immunosuppressive cytokine produced by tumour cells and immune cells that can polarize many components of the immune system. This Review covers the effects of TGFβ on natural killer (NK) cells, dendritic cells, macrophages, neutrophils, CD8⁺ and CD4⁺ effector and regulatory T cells, and NKT cells in animal tumour models and in patients with cancer. Collectively, many recent studies favour the hypothesis that blocking TGFβ-induced signalling in the tumour microenvironment enhances antitumour immunity and may be beneficial for cancer therapy. An overview of the current drugs and reagents available for inhibiting TGFβ-induced signalling and their phase in clinical development is also provided.