Undernutrition and food security have improved in low-and middle-income countries (LMICs) over the last decades. However, food systems in LMICs are now more vulnerable to shocks such as climate change and disease outbreaks which threaten existing gains in nutrition. Diets are key drivers of food systems and nutrition and could be an essential entry point for ensuring food systems’ sustainability and resilience. In a series of six interlinked research papers, this thesis assesses the individual and structural determinants of the effects of food system failures (undernutrition and poor diets) and examines the role that diets could play in ensuring food systems sustainability and resilience in three LMICs: The Gambia, Ghana, and Bangladesh. The first two papers use sub-national data and focus on the determinants of undernutrition and diets of children and adolescents. The studies show that, poor diet diversity, being a male child and maternal short stature (<150cm) strongly predict undernutrition in children. Among adolescents, diets were driven by socio-economic factors (access to pocket money and living in high wealth households) and parental care. Building on the determinants of nutrition and diets of children and adolescents, Papers 3 and 4 focus on national diets and potential vulnerabilities to shocks (climate change, crop and trade failure). The results show that, daily per capita caloric supply is adequate but a high reliance on single crop staples increases vulnerability of food supply and diets to potential shocks compared to more diversified diets. The supply of nutritionally important food groups (fruits, vegetables, nuts, and animal-sources) was suboptimal, showing a “double vulnerability” where the nutrient inadequate diets were also those most vulnerable to shocks. Findings from Papers 3 and 4 informed further investigations into the adherence of national diets to health and environmental sustainability targets (EAT-Lancet dietary guidelines) in low-income settings (Paper 5). While diets were sub-optimal in nutritionally important food groups and high in less healthful food groups (refined grains and sugar), diets were low in food groups known to impact negatively on the environment (beef and lamb, dairy and pork). Finally, Paper 6 identifies key policy reform strategies for food systems to deliver healthy and sustainable diets while being resilient to external shocks (using COVID-19 as a case). Collectively, the research presented in this thesis shows that current food systems do not deliver optimal diets and nutrition and despite increasing food availability, overall diet quality is poor and shows a high vulnerability to external food system shocks. There are opportunities to transform food systems in LMICs to be more resilient to shocks and deliver healthy and sustainable diets.