It is clear that many variables influence estimation of nutrient and energy utilization of individual feed ingredients, including several important factors that were not discussed in this paper (e.g., appropriateness of nitrogen correction of ME for modern broiler genotypes, adoption of net energy). Additionally, continued development and use of predication equations to estimate nutrient digestibility or energy utilization of feed ingredients based on near-infrared spectroscopy and wet-chemistry analyses will only be as good as the underlying in vivo assays on which these equations are based. Refinement and standardization of these assays will also increase the potential for ingredient matrix values to be developed for specific factors such as bird age and health status.