Exercise without proper nutrition can lead to a syndrome called relative energy deficiency in sport (RED-S). Athletes at greatest risk of RED-S are those who restrict intake, exercise for extended periods, and limit the types of foods that they will eat. Early detection of athletes at risk of RED-S is essential to prevent long-term consequences; however, validated screening tools for RED-S are limited. Often, athletes will present with a consequence of RED-S, such as a bone stress injury, amenorrhea, or performance impairments. Initial management for RED-S should be focused on increasing energy availability either by increasing caloric intake or decreasing energy expenditure during exercise. Prevention of RED-S should be a priority for coaches, parents, physicians, and sport organizations, but knowledge on this condition often is limited.