The cytostatic and apoptotic functions of transforming growth factor-β (TGF-β) help restrain the growth of mammalian tissues; loss of these effects leads to hyperproliferative disorders and is common in cancer. However, tumour cells that are relieved from TGF-β growth constraints might then overproduce this cytokine to create a local immunosuppressive environment that fosters tumour growth and exacerbates the invasive and metastatic behaviour of the tumour cells themselves. For these reasons, there is a growing interest in understanding and therapeutically targeting TGF-β-mediated processes in cancer progression.