The view that genes are constrained within somatic cells is challenged by in vitro evidence, and more recently by in vivo studies which demonstrate that mitochondria with their mitochondrial DNA (mtDNA) payload not only can, but do move between cells in tumour models and in mouse models of tissue damage. Using mouse tumour cell models without mtDNA to reflect mtDNA damage, we have shown that these cells grow tumours only after acquiring mtDNA from cells in the local microenvironment resulting in respiration recovery, tumorigenesis and metastasis. Mitochondrial transfer between cells has also been demonstrated following ischaemia‐induced injury in the heart and brain and in lung epithelium, and following lung inflammation. In vitro investigations suggest that stem cells may be mitochondrial donors. The ability of mitochondria to move between cells appears to be an evolutionarily‐conserved phenomenon, relevant to diseases with compromised mitochondrial function including neurodegenerative, neuromuscular and cardiovascular diseases as well as cancer and ageing.