The available energy content of feeds must be known before diets can be formulated, but the difficulty in measuring energy availability requires that estimated values be used. Based on production data, the current system using net energy for lactation (NEL) or total digestible nutrients (TDN) over-valued the energy in feeds by 7 to 8%. A standard discount of 8 TDN units was used for both NEL and TDN. When the discount was calculated from actual intake (4 units per increment of maintenance), feed values for NEL and TDN were overestimated by 5 to 6%. Most feed values for NEL and TDN are estimated from acid detergent fiber. These models do not account for all sources of variation in digestibility, do not allow for variable discounts based on intake and associative effects, and are specific for a given population. Mechanistic models account for many sources of variation and can be modified to include variable discounts based on digestion and passage kinetics. The equation used to convert TDN to NEL does not consider the source of the digestible energy. The NEL of the fat that is contained within feeds is under-valued, and fiber is probably overvalued. Single-component equations are not adequate to describe the complex processes that determine energy availability. The current NEL and TDN systems overvalue the energy in feeds, but both systems still work remarkably well. These systems need to be modified to improve accuracy, but they should not be discarded.