Nature Reviews Clinical Oncology, Published online: 06 February 2026; doi:10.1038/s41571-026-01122-5Platelets can promote cancer progression through either direct and indirect mechanisms, and they can also suppress tumour progression in certain contexts. The authors of this Review discuss how insights on these properties have been leveraged to develop cancer therapies in which platelets are engineered to act as targeted delivery vehicles or as active effector cells, outlining opportunities and challenges for clinical translation.