Over the last century, population transitions linked to development have affected disease and longevity, especially in middle-income countries. In Southeast Asia, the food supply is changing and processed food diets are increasing intakes of energy, fat, sugar, and sodium. This has resulted in diet-related non-communicable diseases (NCDs) that are now major public health problems. Interest in this nutrition transition of changing environment, diet, and lifestyle generated the nutrition labelling research conducted for this thesis. Thai nutrition labelling aims to promote healthier eating among consumers. The labels have not been revised although many complain of difficulty in interpreting the information and we know little about Thai consumer response. This thesis aimed to generate evidence for policies to enhance the utility of future labels in Thailand. Five first author papers resulted (four published and one under review). The first study looks back at food and nutrition labelling in Thailand across the last century and found 81 relevant documents dated between 1908 and 2015. Thai food labelling began to protect consumers from adulterated foods in 1927. Labelling regulations interacted with economic development and international trade in complex ways. The Thai food industry emerged after World War 2 and Thailand joined the international Codex Alimentarius. Consumers became concerned about food and the government created consumer-friendly labels. Over the last two decades, nutrition labelling expanded to promote population health by inducing appropriate changes in eating behaviours and lifestyle. However, domestic protection is now in tension with the rules of the global food trade. The second and third studies were qualitative and used semi-structured in-depth interviews with open-ended questions of a sample of Thai adults. The interview data were obtained by the candidates from 14 university-educated Bangkok members of the Thai Cohort Study (see below) and 20 less educated walk-in supermarket Ranong customers southwest from Bangkok. The second study used a combined Knowledge-Attitude-Behaviour and Health Belief Model to explore participant experiences and motivations in relation to nutrition labels. The third study explored barriers in interpreting information on labels. Participants had difficulties due to low awareness, literacy and numeracy and they thought label formats could improve. The fourth and fifth studies were epidemiological and large in scale. They generated new data on labelling from the nationwide antecedent Thai Cohort Study (TCS) at its 8-year mark in 2013. Cohort members were 42,750 distance learning Sukhothai Thammathirat Open University (STOU) adults under observation since 2005 for multi-faceted research on development-related health-risk transitions. The new TCS data on nutrition label experience and intake of processed foods revealed associations of labels and food choices with socio-demographic factors and with consumption of transitional indicator foods (fourth study) and with elevated lipids or blood pressure or Body Mass Index (fifth study). Labels were most useful for cohort members who read them, understood, and reacted with better health behaviour regarding processed foods. This thesis reveals the challenges of Thai nutrition labels and suggests how to improve label appearance and impact, enhancing utility of future labels and preventing diet-related NCDs.