The present energy evaluation systems of pig feeds are described and evaluated with reference to their scientific basis and practical performance. The different energy evaluation systems are assessed with regard to (i) the definitions and determinations of nutrient fractions in the feed, (ii) the methods for determination of digestible nutrient fractions, (iii) the expression of energy value, (iv) the methods for determination of specific energy coefficients for the different nutrient fractions, and (v) the adequacy of using energy as the sole property in the feed value. It is concluded that present feed evaluation might generally be imprecise due to limitations in the principles of current energy evaluation systems, which oversimplify the nutritional value of the feed and ignore variations in the actual values.