Muscle substrate utilisation during exercise is influenced by availability of endogenous and exogenous substrates, exercise intensity, mode and duration and the training state. This talk will focus on the influence of exercise mode and exercise intensity for regulation of muscle and whole body fat oxidation and how this is regulated. Furthermore the talk will focus on the mechanisms that are currently considered as key for the regulation of muscle fat oxidation. This will include the effect of an intracellular pH decrease attenuating mitochondrial long-chain fatty acid uptake, the potential free muscle carnitine content decrease that limits long-chain fatty acid uptake and the potential decrease in intramitochondrial CoASH content that may  attenuate fat oxidation. Finally the talk will focus on how training mediates an influence on these mechanisms.