In this book we try to link between amazing phenomenon (Autophagy) and Fasting. Autophagy is an intracellular degradation system that delivers cytoplasmic constituents to the lysosome. Despite its simplicity, recent progress has demonstrated that autophagy plays a wide variety of physiological and pathophysiological roles, which are sometimes complex. Autophagy consists of several sequential stepssequestration, transport to lysosomes, degradation, and utilization of degradation productsand each step may exert different function.