Primary Protein-Energy Undernutrition in children is a socioeconomic disease that affects the economically disadvantaged population. In Brazil, it mainly affects low-income children, as their access to food with adequate nutritional values may be scarce or absent. This article aims to explain the etiology, clinical forms, epidemiology, morphophysiological alterations, diagnosis, treatment, approach and prevention of this desease. In addition, the approach to children diagnosed by primary health care in public brazilian health system will be discussed. A bibliographic research was carried out from August to November 2020, using only documents whose method had children undernutrition and its social and morphophysiological correlations in its context, published between 1996 and 2020, in Brazil, including a work published in Spain, in addition to government documents and statistical data from SISVAN, DataSUS and Abrinq Foundation from 2008 to 2020. Studies published in languages other than Spanish and Portuguese, articles with restricted access and studies that did not address specifically the thematic weren't included. As a result, it was found that this pathology affects several systems morphophysiologically with serious consequences for the organism, such as compensatory metabolic changes, immunopathological reactions and possible abnormal neurological development or slowing of child growth. In addition, the importance of monitoring children by health units for the prevention and treatment of primary proteinenergy undernutrition in children has been proven indispensible, as well as the implementation of government policies that contribute to the correct nutrition in day care centers and help families to meet their nutritional needs.