The AMP-activated protein kinase (AMPK) system is a regulator of energy balance at both the cellular and whole-body levels that, once activated by low energy status, effects a switch from ATP-consuming anabolic pathways to ATP-producing catabolic pathways. It now appears to be the major target for two existing classes of drug used to treat type 2 diabetes, i.e., the biguanides and thiazolidinediones. However, in both cases these activate AMPK indirectly, and an interesting question concerns whether a drug that directly activated AMPK would retain the therapeutic benefits of the existing drugs while eliminating unwanted side effects. AMPK activators also now have potential as anticancer drugs.