Book excerpt: Aim Cost is a major determinant of food purchases. The central question for this PhD is whether healthy eating costs more than less healthy eating. Monitoring the cost of foods, meals and diets is important to provide evidence to inform policies. Each approach needs standardised methodologies to ensure fair comparisons over time, and between countries. Method Standardised methods were developed and piloted to measure the relative costs of healthier and less healthy foods, meals and diets using commonly consumed foods and locally collected prices. The use of different data sources and methodological approaches was explored to assess the impact on cost. Results The foods approach measured the price of foods over time using the WHO Europe nutrient profile model and the NOVA classification (degree of processing), and compared the cost of pairs of healthier and less healthy foods. The meals approach compared the cost of popular takeaway meals with healthier home-cooked meals, incorporating the cost of preparation or waiting time. The diet approach compared the cost of a healthy and a current diet. Healthier and less healthy foods, and minimally processed, processed and ultra-processed foods all increased in price at a similar rate over ten years. The healthier items of a pair tended to cost the same as, or more than, the less healthy counterpart. Healthier home-made and home-assembled meals were less expensive than their takeaway counterparts. When time was included, home-assembled meals were the cheapest option and half the home-made meals were at least as expensive as the takeaway meals. The healthy and current diets were similar in price when only the current diet contained alcohol and takeaways, and the energy requirement of the diets was to maintain the recommended BMI for an active person (healthy diet) or maintain the current BMI at current activity levels. Altering aspects of diet, including or excluding takeaways, and generic labels, and altering the energy requirements had considerable impacts on cost. Conclusion This thesis developed novel methods to provide a simple, standardised approach to monitor the price of healthier and less healthy foods, meals and diets enabling comparisons over time, and between countries.