Physical exercise increases muscle glucose uptake, enhances insulin sensitivity and leads to fatty acid oxidation in muscle. The AMP-activated protein kinase (AMPK) is an energy-sensing enzyme that is strongly activated during muscle contraction due to acute decreases in ATP/AMP and phosphocreatine/creatine ratios. Accumulating evidence suggests that AMPK plays an important role in mediating these metabolic processes. Furthermore, AMPK has been implicated in regulating gene transcription and therefore may play a role in some of the cellular adaptations to training exercise. There is also evidence that changes in AMPK activity result in altered cellular glycogen content, suggesting that this enzyme regulates glycogen metabolism. Recent studies have shown that the magnitude of AMPK activation and associated metabolic responses are affected by factors such as glycogen content, exercise training and fibre type. In summary, AMPK regulates several metabolic pathways during acute exercise and modifies the expression of many genes involved in the adaptive changes to exercise training.