Fat and carbohydrate are the primary sources utilised to fuel oxidative, mitochondrial adenosine triphosphate (ATP) resynthesis for human skeletal muscle contraction. The relative contribution of these two substrates to ATP resynthesis and total energy expenditure during exercise can vary substantially, and is predominantly determined by substrate availability and exercise intensity and duration. For example, the increased ATP demand that occurs with an increase in exercise intensity is met by increases in both fat and carbohydrate oxidation up to an intensity of around 70% of maximal oxygen consumption (VO2max) in elite athletes. However, when exercise intensity increases beyond this workload skeletal muscle carbohydrate utilisation is accelerated, which results in a reduction in the relative contribution of fat oxidation to total energy expenditure likely by inhibiting the absolute rate of fat oxidation. As muscle glycogen depletion ultimately results in the inability to maintain exercise at intensities above 70% VO2max, elucidating the limitations to the rate of fat oxidation during exercise is desirable in order to understand, and develop strategies to improve, elite exercise performance. However, despite a considerable accumulation of knowledge that has been gained over the past half century, the precise mechanisms regulating muscle fuel selection and underpinning the aforementioned decline in fat oxidation remain unclear. This presentation will primarily address the theory that a carbohydrate-mediated reduction in the availability of muscle carnitine to carnitine palmitoyltransferase 1 (CPT1), a rate limiting step in mitochondrial fat utilisation, is a key mechanism for the decline in fat oxidation during high intensity exercise. This is discussed in relation to recent work in this area taking advantage of the discovery that skeletal muscle carnitine content can be increased in vivo in humans. Methods to measure skeletal muscle fat utilisation during exercise in humans will also be explored.