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AMPK (AMP-activated protein kinase) is a cellular energy sensor that monitors the ratio of AMP/ATP, and possibly also ADP/ATP, inside cells. Once activated by falling cellular energy levels, it acts to restore energy homoeostasis by switching on catabolic pathways that generate ATP, while switching off anabolic pathways and other processes consuming ATP. AMPK is switched on by increases in AMP via three mechanisms, all of which are antagonized by ATP: (i) promotion of phosphorylation of Thr172 by upstream activating kinases; (ii) inhibition of dephosphorylation of Thr172 by phosphatases; and (iii) allosteric activation of the phosphorylated kinase. Recently, it has been proposed that the first two mechanisms are also triggered by ADP, which might be the physiological signal rather than AMP, and that the third mechanism may not be physiologically significant. We have re-evaluated these questions, and found that only mechanism (ii) is mimicked by ADP, and that ADP is also less potent than AMP, which we still believe to be the primary signal. We have also provided evidence that mechanism (iii), i.e. allosteric activation by AMP, is a quantitatively significant mechanism in intact cells.