Background paper for The State of Food Security and Nutrition in the World 2020 – Price and affordability are key barriers to accessing sufficient, safe, nutritious food to meet dietary needs and food preferences for an active and healthy life. In this study, the least-cost items available in local markets are identified to estimate the cost of three diet types: energy sufficient, nutrient adequate, and healthy (meeting food-based dietary guidelines). For price and availability the World Bank’s International Comparison Program (ICP) dataset is used, which provides food prices in local currency units (LCU) for 680 foods and non-alcoholic beverages in 170 countries in 2017. In addition, country case studies are developed with national food price datasets in United Republic of Tanzania, Malawi, Ethiopia, Ghana and Myanmar. The findings reveal that healthy diets by any definition are far more expensive than the entire international poverty line of USD 1.90, let alone the upper bound portion of the poverty line that can credibly be reserved for food of USD 1.20. The cost of healthy diets exceeds food expenditures in most countries in the Global South. The findings suggest that nutrition education and behaviour change alone will not substantially improve dietary consumption where nutrient adequate and healthy diets, even in their cheapest form, are unaffordable for the majority of the poor. To make healthy diets cheaper, agricultural policies, research, and development need to shift toward a diversity of nutritious foods. This publication was developed as a background study for "The State of Food Security and Nutrition in the World 2020" (available at https://doi.org/10.4060/ca9692en).