This chapter describes the different steps of energy utilization in swine with a description of available energy systems for evaluating the feeds and explores the different components of energy requirements in swine production and the response of growing pigs and reproductive sows to energy intake. It considers some aspects of energy intake and its regulation by feed characteristics, animal characteristics, and environmental factors. For growing pigs, net energy intake is usually calculated as the sum of retained energy at a given production level and the fasting heat production at zero activity. The metabolizable energy content of a feed equals the digestible energy content minus the material energy losses in the urine and combustible gases resulting from fermentation. Diet energy density can be modified by including either fiber-rich ingredients that reduce the energy concentration or fatfiber-rich ingredients that increase the energy concentration.