Exosomes are a subtype of vesicles released by cells of both healthy and neoplastic origin. Preclinical studies suggest a role for tumour-derived exosomes in tumour progression, mainly through the transfer of RNA and proteins from tumour cells to other cells. The transfer of RNA and proteins by tumour-derived exosomes seems to mediate stimulation of angiogenesis and suppression of immune cells; in contrast, exosomes from healthy cells of the immune system appear to have anti-tumour characteristics. Characterisation of the RNA or protein profile of tumour-derived exosomes could have diagnostic or prognostic value, for example, in brain tumours. Anti-tumour therapies could be based on exosomes, for example, by blocking the formation of tumour-derived exosomes or having exosomes release therapeutic agents at specific sites. The most advanced application of this is the use of exosomes from dendritic cells in tumour vaccination; the safety of this has been demonstrated in phase I studies.