As our understanding of stem-cell behaviour rapidly increases, more and more reports suggest that use of stem-cell therapy will extend well beyond regenerative medicine in the near future. Due to their inherent tumoritropic migratory properties, stem cells can serve as vehicles for the delivery of effective, targeted treatment to isolated tumours and to metastatic disease. In vitro, stem cells can readily be engineered by inserting specifically tailored transgenes with antitumour effects to create tumour-seeking therapeutic vehicles. Transgene effects include direct tumour-cell killing, promotion of local immune responses, oncolytic virus production, and prodrug activation schemes. Many of these strategies have been validated in a wide range of studies assessing treatment feasibility or efficacy and establishing methods for real-time monitoring of stem-cell migration and fate in vivo. New insights into avenues for stem-cell sourcing have shortened the probable time to realisation of such treatments for patients. In this Review, we provide an outline of the rationale and status of stem-cell-based treatments for tumours, and we discuss prospects for clinical implementation and the factors crucial for maintaining momentum towards this goal.