The AMP-activated protein kinase (AMPK) is a critical sensor of energy status that appears to have arisen very early during the evolution of unicellular eukaryotes. Its cell-autonomous role in energy homeostasis is conserved in multicellular eukaryotes, but the role of AMPK also appears to have adapted so that it now regulates energy balance at the whole body level (Hardie & Ashford, 2014).AMPK exists universally as heterotrimeric complexes comprising catalytic α subunits and regulatory β and γ subunits. The kinase activity increases >100-fold upon phosphorylation at Thr172 within the α subunit by upstream kinases, which include the tumour suppressor kinase LKB1 and the Ca2+/calmodulin-dependent protein kinase, CaMKKβ. Cellular energy stress caused either by inhibition of ATP synthesis (e.g. during hypoxia or hypoglycaemia) or by acceleration of ATP consumption (e.g. in muscle during exercise) leads to increases in the cellular ADP:ATP ratio, which are amplified by adenylate kinase into even larger increases in AMP:ATP. Displacement of ATP by AMP at multiple sites on the γ subunit leads to activation of AMPK by three complementary mechanisms: (i) promoting Thr172 phosphorylation by LKB1; (ii) inhibiting Thr172 dephosphorylation; (iii) triggering a substantial allosteric activation (Gowans et al., 2013). AMPK is activated by the major anti-diabetic drug metformin, which acts indirectly by causing inhibition of the respiratory chain and thus increasing cellular AMP (Hawley et al., 2010). AMPK is also directly activated (and phosphorylation of Thr172 promoted) by synthetic drugs that bind at a site located between the kinase domain on the α subunit and the carbohydrate-binding module on the β subunit. Ligands that bind this site that occur naturally in mammals have not yet been identified, although the plant product salicylate (the major breakdown product of aspirin) also binds here, causing activation of AMPK and perhaps explaining some therapeutic effects of aspirin (Hawley et al., 2012).Once activated by a cellular stress that causes ATP depletion, AMPK acts to restore energy homeostasis by phosphorylating numerous downstream targets. These either switch on catabolic pathways generating ATP, or switch off processes consuming ATP, including lipid, polysaccharide and protein biosynthesis. For example, AMPK switches on fatty acid oxidation acutely by phosphorylating acetyl-CoA carboxylase-2 (ACC2) and thus lowering malonyl-CoA, and in the longer term by promoting mitochondrial biogenesis and expression of oxidative enzymes. At the same time, it switches off fatty acid synthesis acutely by phosphorylating and inactivating acetyl-CoA carboxylase-1 (ACC1), and in the longer term by inhibiting transcription of fatty acid synthesis enzymes. To examine the importance of the acute regulation of ACC, mice with double knock-in (DKI) substitutions that prevent their phosphorylation by AMPK (ACC1-S79A, ACC2-S212A) were generated (Fullerton et al., 2013). While not obese, these mice had elevated di- and tri-acylglycerol in liver and muscle, displayed signs of fatty liver, and became severely glucose-intolerant and insulin-resistant. These metabolic parameters deteriorated in wild type but not DKI mice that were fed a high-fat diet, so that they became similar to those of the DK1 mice. However, while the metabolic parameters all improved when the wild type mice were treated with metformin, those of the DKI mice did not. These results suggest that the insulin-sensitizing effects of metformin are mediated entirely by phosphorylation of ACC1 and ACC2 by AMPK, with an associated reduction in lipid storage.In addition to these cell-autonomous effects, the AMPK system also mediates energy balance at the whole body level, particularly via effects in the arcuate nucleus of the hypothalamus. The hormone ghrelin, a “hunger hormone” released from the gut during fasting, activates AMPK via GSHR2 receptors and the Ca2+/CaMKKβ pathway in presynaptic neurons upstream of agouti-related protein (AgRP) expressing neurons (Yang et al., 2011). This leads to activation of the AgRP neurons, causing an orexigenic effect. On the other hand, the “satiety” hormone leptin, released from adipose tissues that have adequate triacylglycerol stores, inhibits AMPK in the same neurons and thus causes an anorexigenic effect (Yang et al., 2011). It has been proposed that insulin and leptin (the latter most likely acting indirectly via release of an opioid such as β-endorphin) both inhibit AMPK in these neurons by activation of the PI 3-kinase-Akt-S6K1 pathway, with S6K1 phosphorylating AMPK at a site that inhibits its phosphorylation and activation by LKB1 (Hardie & Ashford, 2014; Hawley et al., 2014).