The physical growth of young children in low- and middle-income countries is reduced compared to international standards. The deviations in growth in both weight and height are greatest in the first 2 years of life and this has serious consequences for child mortality, development, adult stature, and health. The determinants of these patterns of growth faltering include intergenerational factors, such as maternal height, short birth interval, and conditions in pregnancy, including maternal underweight and anemia. These factors contribute to fetal growth restriction and premature delivery, which put many infants on a different growth trajectory. Postnatal exposure to microbes resulting in diarrhea and febrile infectious diseases and poor quality diet further compromise growth. Determinants of growth faltering after birth vary by setting and are not independent of each other. For example, the adverse effects of diarrhea on growth may be mitigated by a high-quality diet. Global estimates suggest that 25% of stunting can be attributed to fetal growth restriction and even more in countries in South Asia with a high prevalence of low birth weight. Infectious diseases may contribute a similar amount and subclinical enteric infections can result in intestinal dysfunction with adverse effects on nutrition and growth. Dietary factors, especially consumption of complementary foods of insufficient quality, have a paramount role in growth faltering in the critical period of infancy.