This book provides new knowledge about the nutrition and metabolism, as well as the roles of nutrients in the immunity, health, and management of companion animals (dogs and cats). The domestic dog (facultative carnivore) and the domestic cat (obligate carnivore) have evolved differentially in their digestive tract, nutrition, metabolism, chemical sensing, and feeding behavior. These animals have been human companions for at least 12,000 and 9,000 years, respectively, and continue to contribute to the mental health and well-being of children, adolescents, and adults. Both dogs and cats have become increasingly popular in many countries and worldwide over the past decades. Comprehensive update about how the animals utilize dietary nutrients for optimum growth, development, and health is beneficial for their owners, as well as students and researchers. Chapter 4: Characteristics of Nutrition and Metabolism in Dogs and Cats is available open access under a Creative Commons Attribution 4.0 International License.