The diagnosis and management of failure to thrive, a multifactorial condition, can be a challenge for the primary care provider. This article deals with organic failure to thrive and nonorganic failure to thrive in ambulatory settings. The complex etiology of failure to thrive is addressed relative to maternal/paternal and infant/child characteristics. Physical assessment is addressed with special attention on critical growth measurements, feeding and eating paterns, developmental delays, and psychosocial issues. Interventions in the areas of nutrition, development, and psychosocial interactions are examined with an emphasis on team management. Long-term effects of failure to thrive and primary and secondary preventive interventions are addressed.