Background: Excess male morbidity and mortality is well recognised in fields like neonatal medicine and evolutionary biology. In contrast, within the global nutrition community, it is less understood and often stated that girls are most at risk of nutritional deficits. With a focus on a clinical, biological, evolutionary, and social perspective, this PhD sets out to explore the evidence for sex differences in the epidemiology of undernutrition, the reasons for these differences and whether and how these might need to be addressed in policy and practice. Methods: A systematic review and meta-analysis examined undernutrition-specific estimates for wasting, stunting and underweight by age and sex using a random-effects model. A qualitative synthesis reviewed how sex differences were reported and explained within studies. A narrative review explored which early life mechanisms might underlie these differences. A further random effects meta-analyses described mortality risk associated with anthropometric deficits (wasting, underweight and stunting) in children 6–59 months by age and sex in multi- country cohort data. A pooled secondary analysis assessed treatment outcomes by age and sex. Results: Boys are more likely to be wasted (pooled OR 1.26, 95% CI 1.13 to 1.40), stunted (pooled OR 1.29 95% CI 1.22 to 1.37) and underweight (pooled OR 1.14, 95% CI 1.02 to 1.26) than girls with variations in differences by regions and age groups. A complex interaction of social, environmental, physiological, and genetic factors likely underlies these differences throughout the life cycle. Sex differences appear to be more pronounced in early infancy, in more severe presentations of malnutrition and in more fragile contexts. For wasted children, there is no difference in mortality risk between children 6–23 months and children 24–59 months (pooled RR 1.08, 95% CI 0.52-2.22, p=0.826 for MUAC <125 mm; RR 1.35, 95% CI 0.79-2.33, p=0.272 for WHZ <−2). For underweight and stunting, younger children had a significantly higher risk of mortality than older children (underweight - pooled RR 2.57, 95% CI 1.65–4.00, p<0.001; stunting - pooled RR 2.83 95% CI 2.09–3.82, p<0.001). Despite a higher risk of wasting, stunting and underweight in boys, in pooled analysis for each anthropometric deficit, we found no differences in mortality risk between girls and boys. In wasting treatment programmes, we observed very few differences between girls and boys in treatment outcomes but have highlighted the need for future research that considers the effect of health and social care indicators. Conclusions: The risk of undernutrition differs according to sex and the extent and direction of differences is greatly influenced by age and context. This highlights the need to improve data collection in programmes, surveys and research through the full disaggregation and analysis of sex and age to identify which children are most vulnerable in specific contexts, and to allow comparison of programme data with population-level burdens. Ultimately this research aims to contribute to a better understanding of risk so that nutrition interventions, and resources can be targeted according to need