Fetal, suckling, and postweaning vertebrates have distinct dietary inputs that impose different functional demands on the developing intestine. Differences between species and life-history stages in intestinal structure and functions are set by genetic determinants that reflect evolutionary diets. Dietary inputs interacting with genetic determinants influence intestinal functions by triggering the production of new enterocyte populations and (or) by reprogramming existing enterocytes. In addition to nutrients, nonnutritive components of amniotic fluid and milk, such as growth factors and hormones, are important mediators of intestinal development and in humans can exert influences at as early as 10 weeks of gestation, when fetuses begin swallowing amniotic fluid. Changes in diet composition during suckling elicit limited and apparently nonspecific responses in intestinal structure and activities of brush-border hydrolases and transporters. The ability to adaptively modulate intestinal brush-border functions develops at weaning, when diet composition begins to vary unpredictably. Hydrolytic and transport capacities of the developing intestine are matched to age-related increases in dietary inputs, are not in great excess, and may be growth limiting. Although not as extensively studied, postnatal diet also influences development of intestinal endocrine and immune functions and has a complex, poorly understood interaction with the developing microflora.