The aim of this Special Issue was to publish high-quality papers on poultry nutrition and the inter-relations between nutrition, metabolism, gut microflora, and poultry performance. Therefore, we present here original research articles in the following topics: - the nutrient requirements of poultry; - the impact of dietary fibre on poultry health and performance; - the use of feed additives to improve gut microflora and performance; - the effect of feeding on poultry meat and egg quality; - factors that cause variation in feed ingredients used in poultry diets; - alternative feed ingredients for use in poultry feed formulations; - the impact of gastrointestinal pH on nutrient bioavailability and the intestinal microbiota in poultry; - phytate and xylan 'susceptibility' in relation to the effects of enzymes; - factors that influence reactivity and enzyme efficacy; - in vitro models to determine the digestibility of poultry diets.