Non-alcoholic fatty liver disease (NAFLD) affects up to 1 in 4 people globally and predisposes to type 2 diabetes, heart disease and liver disease. The mechanisms underlying NAFLD remain unclear. Here, we study whether dysfunction of the cellular ‘energy motors’ – mitochondria – is a key mechanism underlying NAFLD in humans. To this end, we use state-of-the-art stable isotope methods to study hepatic mitochondrial function in individuals with and without NAFLD in response to modifiable risk factors and to currently available treatments. We also use large population-based cohorts to study whether genetic variants predispose individuals to hepatic mitochondrial dysfunction, and whether those individuals are also predisposed to NAFLD. Finally, we investigate in mice whether a pharmacological increase in mitochondrial function can ameliorate NAFLD. This project is expected to provide important new knowledge on the pathogenesis of NAFLD, which may help to develop new pharmacotherapies and to identify individuals who are at a particularly high risk of NAFLD.