Food security and sustainability are paramount goals of global policy in the Twenty First Century. What are the contributions of applied ethology and animal welfare in delivering these major goals? Livestock plays a major role in food security and nutrition (FSN), because meat and animal products are important in people’s diets and the livestock sector is central to food systems’ development. And many aspects of the welfare of livestock are important for production, as expressed in the advice, ‘Look after your animals and they will look after you,’ for example, reduction of disease. Consideration of animal welfare can enable advantages for production that have not otherwise been identified, and is therefore not just compatible with but important for achieving good management of livestock for production, livelihoods and FSN. Furthermore, understanding of animal behaviour and care for livestock welfare contribute to all three pillars of sustainable agriculture: economic profitability, social equity and environmental health. Ethology clarifies the interactions between animals, humans and the environment, and thus helps to optimise livestock management for the best balance between these complex priorities. However, although considering behaviour and welfare is valuable for production and FSN, people do not always recognise or act on this value. They may need information or help to do so: mechanisms are needed to provide information, help and sometimes financial assistance to producers. These mechanisms need input from experts in applied ethology and animal welfare, at both practical and policy level. Six policy areas are briefly outlined. Each requires (to a variable degree) understanding of animal behaviour and welfare and implementation of that understanding. (1) Development of humane, sustainable food security strategies; (2) Promotion of humane, sustainable livestock systems; (3) Reduction of livestock reliance on human-edible arable crops, especially cereals; (4) Development of specific food and livestock policies to assist vulnerable sectors of the population; (5) Sustainable diets; and (6) Development of markets for humane, sustainable livestock production. Urgent efforts are needed to achieve food security and sustainability, by producers, governments and intergovernmental organisations in consultation with other stakeholders including scientists and non-governmental organisations. The livestock sector has both positive and negative impacts. Consideration of animal behaviour and welfare helps to ensure and increase the positive impacts, and to reduce and avoid the negative, and should therefore be a priority for countries worldwide.