This document defines failure to thrive and outlines its causes and evaluation process. Failure to thrive is defined as weight or growth below major percentiles. Causes include inadequate intake due to issues with appetite, ingestion, or food availability; altered growth regulation from prenatal or genetic factors; calorie wasting from vomiting or malabsorption; and increased caloric needs. Evaluation involves assessing growth data, medical history, nutrition, development, a feeding observation, and labs if indicated. Hospitalization may be needed if abuse is suspected or for severe cases.