ATP-sensitive potassium (K_ATP) channels are metabolic sensors that couple the metabolic state of the cell to the electrical activity of the plasma membrane. They consist of pore-forming Kir6.2 and regulatory sulphonylurea receptor (SUR) subunits. Metabolic regulation of channel activity is mediated by changes in intracellular adenine nucleotides: ATP closes the channel by binding to Kir6.2, whereas interaction of Mg-nucleotides (MgATP, MgADP) with the nucleotide-binding domains of SUR1 stimulates channel activity and reverses channel inhibition by ATP. Channel activity thus reflects the balance between these excitatory and inhibitory effects. K_ATP channels are found in numerous tissues. In neurones they regulate electrical activity in response to glucose, neuropeptides and ischemia; in heart they are important for ischemic preconditioning and the stress response; in smooth muscle they regulate vascular tone and in endocrine cells they mediate hormonal secretion. Our studies focus on their role in insulin secretion. At rest, K_ATP channels are open, hyperpolarizing the pancreatic beta cell membrane and inhibiting insulin release. When plasma glucose levels rise, K_ATP channels close, depolarizing the beta-cell and opening voltage-gated Ca²⁺ channels. The resulting Ca²⁺ influx triggers insulin release. The sulphonylurea drugs used to treat type 2 diabetes stimulate insulin secretion by binding to, and closing the K_ATP channel. Gain-of-function mutations in the genes encoding both Kir6.2 (KCNJ11) and SUR1 (ABCC8) can cause neonatal diabetes. Some mutations produce a severe clinical phenotype, characterized by developmental delay, epilepsy, muscle weakness and neonatal diabetes (DEND syndrome). In many patients, sulphonylureas can successfully be used to treat their diabetes, and in some individuals the neurological symptoms can also be alleviated. This lecture will discuss the mechanisms by which nucleotides modulate K_ATP channel activity, how mutations causing human disease alter K_ATP channel function, how alterations in K_ATP channel activity cause the disease phenotype in man and mouse, and why some mutations are susceptible to sulphonylurea therapy and others are not.