Energy is not a nutrient, but a property of nutrients to “perform work”. Not all nutrients in the diet are used by the animal for this purpose (and eventually lost as heat), as nutrients will also be retained by the animal and in animal-derived products, and lost in the excreta. Energy systems, which assign a value to a feed that can be compared with the requirement of the animal, are a compromise between properties of the diet and the animal. Some energy systems have properties that are closer to the diet (DE) whereas others are closer to animal (NE), which leaves us with the question what the most appropriate compromise is. The NE system has been widely used by pig nutritionists. The system was developed more than 100 years ago, but research carried out between the 1970s and 1990s made practical implication in pig nutrition possible. Different reasons contributed to the adoption of the NE system, especially in Europe. First, a wide range of feed ingredients with different nutritional characteristics is used in Europe and an energy system should to be able to account for this diversity. Second, the impact of animal production on the environment has led to legislation and opportunities to reduce the protein content of the diet, and it became important to assess the energy value of carbohydrates and free amino acids relative to that of (excess) protein. Also, the NE intake appeared to be a better “predictor” of performance than the ME or DE intake. The AMEn system is widely used by poultry nutritionists. Because it “corrects” the AME value for zero nitrogen retention, it assigns a reduced energy value to protein. Animals retain nitrogen in the body or in animal-derived products and correcting the AME for zero nitrogen retention has little to do with biological reality. Dietary protein also has a lower energy value in a NE system, but for very different and more appropriate reasons. First, there is an energy cost (as ATP) for synthesize protein from absorbed amino acids, but also to synthesize urea or uric acid from excess protein. It thus costs energy to retain protein, but it also costs energy to excrete nitrogen from excess protein. Although the AMEn may provide similar (numerical) values as NE, it does so for the wrong reasons. Energy systems are not static and remain a compromise, and specific aspects may need to be considered related to the diet (e.g., the energy value of an enzyme), the animal (e.g., its digestive capacity), or the environment (e.g., the effect of heat stress). A solid and biological basis of a system is necessary to properly account for these aspects.