Book excerpt: The major emphasis in this book is a compilation and definition of what is known about components of human milk, including glycoconjugates, that inhibit common pathogens of the infant. Also discussed are other bioactive constituents whose relevant biological roles are also beginning to be defined. Hormonal and cytokine activity, immunomodulating and autoinflammatory agents, xenobiotics, and conditionally essential nutrients in milk could have roles in the protection of the infant, but may also participate in digestive processes, maternal--infant communication, maturation of the gut, central nervous system, and other components of infant growth and development. Like the protective activities, these are discussed in terms of their presence in milk, structures, potential functions, and structure/function relationship. Components whose role is nutritional support during early development of the infant are also included.