Observational and experimental studies indicate a remarkably consistent association between rapid growth and weight gain during infancy and higher risks for obesity in later childhood and adult life. This association appears to be equally relevant to breastfed and formula milk-fed infants, and infants small for gestational age and with normal birth weight. The type of infant milk feeding, energy intake, and milk nutrient composition are important determinants of infant growth and weight gain. There is also accumulating evidence that genetic factors related to adult obesity susceptibility act in the central nervous system to regulate intrinsic levels of infant appetite and satiety, and they impact on infant dietary behaviors to influence growth and weight gain. These genetic factors indicate an early life trajectory to later obesity that starts with rapid infancy gains in weight, length, and fat and lean mass, before the subsequent emergence of high BMI and adiposity. Better understanding of the anthropometric, metabolic and behavioral correlates of this trajectory will help to enable early-life prediction and preventive strategies against obesity and related metabolic disorders.