The level of energy in a pig’s diet influences the rate and efficiency of gain, the quality of the resulting carcass and even the quality of the pork produced from the carcass. However, energy requirements and energy use in pigs are not well understood, due to the complexity of the topic. Unlike nutrients in the diet, energy is supplied by numerous dietary constituents, and each of these is utilized as an energy source in different ways and in accordance with their metabolic fate. This chapter will discuss the basics of energy metabolism, energy systems, energy sources, energy requirements and methods to improve the utilization of energy by the pig.