Development in agricultural sciences, particularly in farm animal sciences, resulted in the increased productivity to meet the demand for high quality and relatively cheap protein sources for human nutrition. In parallel, this increased productivity challenges the adequate supply of nutrients, including protein and energy, needed to cover not only high performances, but also insure animal health and welfare, reproduction and quality of products in a sustainable environment. The precise understanding of the animal biology is crucial for animal health and welfare, sustainable animal production, and health of animal product consumers. This book focuses on combining basic and applied research and its practical applications. To achieve these goals, many important topics are presented and discussed in detail. The most important issues in this book are: physiological aspects of protein and energy metabolism and nutrition, animal health and welfare metabolic related issues, effect of feeds and feed processing on energy and protein digestion and metabolism, methodological aspects of research on protein and energy metabolism, environment protection and enhancement of the quality and health-promoting features of animal products. This book constitutes a good source of knowledge for those who like to be up to date with the newest trends and findings in energy and protein metabolism in farm animals.