Human milk provided by healthy and well-nourished mothers is believed to cover the infant's nutrient requirements during the first half year of life. It is composed of a mixture of nutritive components as well as other bioactive factors with relevant physiologic effects in the neonate infant. Human milk composition has a dynamic nature and varies with time postpartum, during a nursing, and with the mother's diet and certain diseases. The changes of human milk composition with time of lactation seem to match the changing needs of the growing infant over time. Human milk proteins are a source of peptides, amino acids, and nitrogen for the infant, but also in the protein fraction reside other properties of human milk that may benefit the breastfeeding infant. Specific whey proteins are involved in the development of the immune response (immunoglobulins), whereas others participate in the nonimmunologic defense (lactoferrin). In addition, human milk contains a complex mixture of oligosaccharides that are present only in minute amounts in other mammal's milk. They may act as inhibitors of bacterial adhesion to epithelial surfaces, and thus play an important role in preventing infectious diseases in the newborn infant. Oligosaccharides may also promote the development of a so-called bifidus flora. In the next years, future research will lead to improved characterization of human milk components and elucidation of their individual mechanisms of action, which will increase our knowledge about the properties of human milk and the benefits of breastfeeding for the infant.