To better understand the complex associations between undernutrition and poverty in determining behavioral outcome for infants with failure-to-thrive (FTT), we have reviewed findings from research in undernutrition among animal models and among children in developing countries, where rates of infant undernutrition are extremely high. The associations among undernutrition, poverty, and family functioning persist in both animal and human research, whether manipulated in laboratory settings or observed in natural settings. Although environmental support and stimulation appear to ameliorate many of the negative consequences associated with undernutrition, infants with a history of nutritional deprivation are at increased risk for behavioral and emotional problems. Recommendations for prevention and intervention follow an ecological framework and include adequate access to food, knowledge of nutritional requirements and feeding approaches, support for parents and families, and a nurturant environment for infants.