Failure to thrive (FTT) has been defined in a number of ways, but most definitions include a weight less than the 5th percentile on the growth chart or a decreasing rate of weight gain. Nonorganic failure to thrive (NOFTT), i.e., FTT not due to organic disease, is the most common category of FTT in the United States and is associated with delayed growth and development and abnormal behaviors. Factors extrinsic to the infant are primarily responsible for NOFTT. That acute undernutrition may be a cause of the poor weight gain is suggested by anthropometric studies and by the observation that NOFTT infants often gain weight when food is supplied. Yet, decreased caloric intake has been documented in only a few infants, and not all infants immediately gain weight when given adequate calories. Current thinking attributes lack of weight gain in NOFTT to probably mixed interacting causes, including decreased nutrition, and abnormal hormonal mechanisms associated with abnormal behavior. That behavior and nutrition are related is recognized, but their interactions have not been adequately documented or explained. It is unknown whether behavior affects growth directly through nutrition or independently of nutrition. Until the cause or causes of poor growth and development in NOFTT are understood, permanently reversing the process will be difficult. This report reviews what is presently known about nutrition and growth in NOFTT.