By six months of age, the infant’s capacity to digest and absorb a variety of dietary components as well as to metabolize and excrete the resulting products is near the capacity of the adult. Consideration of the long-term effects of inadequate or excessive intakes during infancy now assumes greater importance. These considerations about delivery of adequate amounts of nutrients are the basis for many of the feeding practices advocated during the second six months of life. Although it is clear that all nutrient needs during this period can be met with reasonable amounts of currently available infant formulas, addition of other foods after four to six months of age is recommended. In contrast, the volume of milk produced by many women may not be adequate to meet all nutrient needs of the breast-fed infant beyond about six months of age, especially iron. Thus, for breast-fed infants, complementary foods are an important source of nutrients. Complementary foods (i.e., the additional foods, including formulas, given to the breast-fed infant) or replacement foods (i.e., food other than formula given to formula-fed infants) should be introduced step by step to both breast-fed and formula-fed infants, beginning between four and six months of age.