AMPK is a ubiquitous sensor of cellular energy status in eukaryotic cells. It is activated by stresses causing ATP depletion and, once activated, maintains energy homeostasis by phosphorylating targets that activate catabolism and inhibit energy-consuming processes. Evidence derived from non-mammalian orthologs suggests that its ancestral role was in the response to starvation for a carbon source. We review recent findings showing that AMPK is activated by ADP as well as AMP, and discuss the mechanism by which binding of these nucleotides prevent its dephosphorylation and inactivation. We also discuss the role of the carbohydrate-binding module on the β subunit and the mechanisms by which it is activated by drugs and xenobiotics such as metformin and resveratrol.