The changes that have taken place in broiler and pig genotypes over the past fifty years have had significant repercussions for nutritionists, as well as for the managers of these species, and this paper begins by exploring those changes as a means of predicting the nutritional and environmental requirements of broilers and pigs in the future. The proportion of the daily requirement that is attributable to maintenance has been reduced, due to the increased growth rates and the successful selection for feed efficiency. As a result, the amino acid balance in the feed required to meet these requirements has changed markedly over the years. Similarly, the amino acid requirements at an age have increased in the early growth period, and decreased towards the time when the birds and pigs are harvested. These changes have only been recognised through the use of simulation models. But in addition to changes in genotypes has been a considerable improvement in our knowledge of the nutrient and anti-nutritional contents of feed ingredients available to the monogastric industry, as well as of methods whereby more benefits may be extracted from feed ingredients by means of improved processing and the addition of feed enzymes. Because of the importance of these changes to the efficient feeding of broilers and pigs, some of these changes, and their implications for monogastric nutrition in the 21st Century, are considered in this paper.