Mitochondria are the powerhouses of eukaryotic cells. Through oxidative phosphorylation (OxPhos), mitochondria extract energy from nutrients and use it to transfer protons across a proton-impermeable membrane creating a proton motive force to drive ATP production. Since the enzymes that catalyse OxPhos are encoded in both the mitochondrial and nuclear genomes, mutations in either one can cause mitochondrial disease, with a minimum prevalence of 1 in 5,000 live births in the EU. The molecular-biochemical causes of mitochondrial disorders and their clinical manifestations are highly diverse, but the most common defects are in complex I, the first enzyme of the OxPhos system. Currently, the mechanisms linking specific complex I mutations to OxPhos failure, cell and tissue damage, and ultimately disease, are poorly understood – precluding development of rational, evidence-based, therapies. The paucity of tools available to define mitochondrial function in the living cell is a major hindrance.During my research in the United States, I have developed optical instrumentation, technology and expertise capable of defining the bioenergetic status of mitochondria in living cells, with an exquisite level of accuracy, under tightly defined conditions. Every bioenergetic parameter that impacts on OxPhos function is accessible, including the redox potential of both substrates of complex I, the proton motive force, and the flux. Here, I propose to transfer my unique instrumentation and expertise in cellular bioenergetics to the Mitochondrial Biology Unit in Cambridge, UK, and integrate them with existing projects on molecular and clinical aspects of mitochondrial disorders, in order to link the biochemical definition of complex I dysfunctions with the characterization of their consequences in vivo, provided by my technology. This integrated approach will allow us to bridge fundamental gaps in knowledge, and drive forward the development of effective clinical therapies.