Adenosine levels are increased under conditions of energy deprivation, both because intracellular energy stores are reduced and because ATP is released. The adenosine thus formed can serve to influence energy homeostasis in a number of different ways, besides alterations in blood supply and cellular work (including contraction, maintenance of membrane potential, and biosynthesis), which will be covered in other chapters. Here, effects on energy homeostasis will be briefly reviewed. Adenosine acting at the A(1) receptor is a powerful and nonredundant inhibitor of lipolysis. It increases glucose uptake in fat and muscle, but its effects on insulin secretion may be even more important than the actions at insulin target tissues. Glucagon is also influenced. In addition to these peripheral actions, adenosine acts in the brain to regulate sleep-wakefulness, food intake, and body temperature. These effects are both direct at the relevant neurons and indirect by influences on regulatory transmitters and hormones.