Failure to thrive affects the lives of many infants and young children at critical times in their development and represents a significant public health problem in the United States. Moreover, this condition is invisible and can affect children for long periods of time before it is recognized. The long-term psychosocial sequelae of failure to thrive have only begun to be recognized but may be more severe than first realized. We do know that the costs to society in terms of acute pediatric hospitalization and long-term rehabilitation, foster care, and mental health treatment of young children who present with failure to thrive are considerable. Children who are diagnosed with failure to thrive represent a special challenge and opportunity for intervention, especially preventive intervention, because it is quite possible that many of the long-term consequences of this condi tion on psychological development can be lessened via early recognition and intervention. However, the potential for preventive intervention in failure to thrive has been limited by the state of the art in scientific knowledge and practice. Despite the frequency with which failure to thrive is encounter ed in ambulatory and inpatient settings, there is little scientific infor mation to guide practitioners. Research on the causes and consequences of failure to thrive has been very much limited by small sample sizes, lack of common definitions, and short follow-up periods. Uncertainties in the science of failure to thrive coincide with the considerable practical difficulties involved in diagnosis and inte~vention.