Undernutrition is a form of malnutrition characterized by an insufficient dietary intake to sustain good health in an individual. Undernutrition is most commonly described in terms of wasting (low weight for height), stunting (low height for age), and underweight (low weight for age). The etiology of malnutrition is complex and includes multiple individual, social, and environmental factors. Healthy nutrition starts at birth with the introduction of breastfeeding, appropriate complementary feeding at 6 months of age, followed by a dense and diverse diet throughout childhood and the adolescent period. Nutritional intervention for the treatment of undernutrition focuses on support for breastfeeding during infancy, supplementary feeding with infant formula in case of lack of human milk, inpatient rehabilitation of complicated severe acute malnutrition, and the use of ready-to-use therapeutic foods (RUTFs) for outpatient management of uncomplicated severe wasting and supplementary foods for the treatment of moderate wasting. Micronutrient supplements are recommended for vitamin A, iron, and iodine deficiency, and new products with multiple micronutrients have been studied. More research is needed to address the nutritional needs of adolescents, and there is a general lack of data on nutritional interventions for this age-group.