The world is witnessing major shifts in dietary patterns and – in parallel – the threat to agricultural biodiversity. The implications for human health and the resilience of our food systems are significant. Agricultural landscapes are becoming increasingly simplified as the number of crops and crop varieties grown on farms declines. Responding to a variety of pressures, farmers have replaced many time-tested local varieties with a small number of modern ones – a pattern which holds true for both food crops and animal breeds. Coinciding with the threat to agricultural biodiversity has been a trend towards the homogenisation of diets. Today 30 crops supply 95 per cent of the calories that people obtain from food, and only four crops – maize, rice, wheat and potatoes – supply over 60 per cent. This is noteworthy given that over the millennia, humans have domesticated or collected approximately 7,000 species of plants for food. Such heavy reliance on an unprecedented narrow range of crops, crop varieties and animal breeds brings long-term and increasing risks for agricultural production, for biodiversity, for livelihoods, and for nutrition. It also undermines the ability of agriculture to adapt to climate change. This synthesis paper discusses why agricultural biodiversity and dietary diversity are important, the relationship between them, the reasons why they are at risk, and what can be done to foster them. It calls for action to reverse these trends, in order to put diversity back in our farming systems and on our plates and to preserve it where it still exists. Drawing on literature review, action research, innovation and multi-stakeholder platforms in Indonesia, Uganda, Zambia and Bolivia as part of the Sustainable Diets for All Programme coordinated by Hivos, IIED and partners, the paper aims to inform policymakers, agriculturalists and civil society working on these issues.