Various mechanisms and devices can facilitate coupling between energy-producing process and energy requiring processes. For example, in living systems, synthesis of adenosine triphosphate requires energy and therefore must couple with an energy providing process of electron transport chain. On the other hand, the hydrolysis of adenosine triphosphate supplies the energy needed in living systems. This chapter discusses bioenergetics briefly as a representative analysis of energy coupling in biochemical cycles, and energy expenditure.