Energy is by far the most expensive component of the diet and, as such, influences the cost of feeding pigs more than any nutrient. Apart from its expense, energy is a critical topic because the level of energy in the 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. Unfortunately, as important as energy is, it is not a well-understood topic, due to its complexity. Unlike nutrients in the diet, energy is supplied by numerous dietary constituents, namely amino acids; lipids; simple sugars arising from starch, lactose and sucrose; and complex carbohydrates more commonly known as fibre. Each of these is utilized by the pig as an energy source in different manners, and, indeed, the variation in utilization is also influenced by its metabolic fate.