Horizontal transfer of mitochondria is the movement of whole mitochondria and mitochondrial DNA between cells. Mitochondria from donor cells are transported and incorporated into the endogenous mitochondrial network of recipient cells contributing to changes in the bioenergetics profile and in other functional properties of recipient cells. Horizontal cell-to-cell transfer of mitochondria and mitochondrial genome can occur among mammalian cells in vitro and in vivo. Mitochondrial transfer supports the exogenous replacement of damaged mitochondria, thereby rescuing mitochondrial defects. Stem cells, immortalized cells or primary cells are usually used as mitochondrial donors in most studies. These cells may transfer mitochondria to surrounding cells in their niche, thus affecting cell differentiation, proliferation, tissue homeostasis, development and ageing. Mechanism Horizontal transfer of mitochondria is mediated by actin-rich membrane protrusions named tunneling nanotubes (TNTs). The establishment of a nanotube begins with the formation of a filopodium-like membrane protrusion that retracts after reaching the recipient cell, leaving an ultrafine structure that is separated from the substrate. Chemical inhibitors or mechanical stress impairs the formation of TNTs and reduces mitochondrial exchange. On the other hand, certain types of stress agents such as doxorubicin or ethidium bromide increase TNT formation. Other proposed mechanisms of transfer include membrane microvesicles, cell fusion or mitochondrial extrusion. In vitro transfer The first evidence of functional mitochondrial transfer in vitro has been documented between human mesenchymal stem cells (hMSCs) and human lung carcinoma cells. Healthy mitochondria from hMSCs moved to recipient lung carcinoma cells with nonfunctional mitochondria and repaired their function. Intercellular transfer of mitochondria in culture has been documented from MSCs and endothelial cells to breast cancer cell lines, ovarian cancer cell lines or to osteosarcoma cell line. Mitochondrial transfer can occur also between cancer cells such as mesothelioma and laryngeal carcinoma cells. Non-tumor cells such as human renal epithelial cells, human retinal pigment epithelial cells or human monocyte-derived macrophages have been shown to transfer their mitochondria as well. All these data suggest that this phenomenon, regardless of the exact mechanisms involved, may be a fundamental physiological process well worthwhile exploring in a whole organism setting. In vivo transfer One of the first evidences of in vivo horizontal mitochondrial gene transfer was found in a transmissible canine venereal tumor (CTVT), highly adapted cancer transmitted during mating of feral dogs. Phylogenetic analyses of mitochondrial sequences revealed that CTVT cells periodically acquire mitochondria from its host and ensure overcoming high mutation rate that would