Children with very low weight for age or height and those who do not maintain an appropriate growth pattern may have failure to thrive (FTT), also known as weight faltering. If confirmed by repeated valid measurements, FTT should prompt a search for causes of undernutrition, including neglect, family food insecurity, and underlying medical conditions. Inadequate caloric intake is the most common cause of FTT, but inadequate nutrient absorption or increased metabolism is also possible. Difficulty attaining or maintaining appropriate weight is the first indication of FTT, and sustained undernutrition can impede appropriate height, head circumference, and the development of cognitive skills or immune function in extreme cases. Early identification and management of the issues causing undernutrition are critical. In most cases, an appropriate growth velocity can be established with outpatient management based on proper nutrition and family support. Primary care physicians can effectively treat most children with FTT, and subspecialist consultation or hospitalization is rarely indicated.