This report aims to support countries in the necessary transition toward healthier, more sustainable diets by integrating biodiversity in food-based interventions to support nutrition and health. It is intended to help guide decision-makers in the health, nutrition and other sectors, to: Consider the important role of biodiversity in food systems for the development of integrated interventions to support healthy, diverse and sustainable diets; To focus investments and country support for more comprehensive, coordinated and cross-cutting public health and nutrition projects and policies; and To strengthen the resilience of food systems, health systems, and societies, each of which are each increasingly compromised by widespread ecological degradation, biodiversity loss and climate change. Biodiversity at every level (genetic, species and ecosystem level) is a foundational pillar for food security, nutrition, and dietary quality. It is the basic source of variety in essential foods, nutrients, vitamins and minerals, and medicines, and underpins life-sustaining ecosystem services. It is a core environmental determinant of health, often a vital ingredient of healthy nutritional outcomes and livelihoods, gender equality, social equity, and other health determinants. Biodiversity can play a more prominent role in planning for nutritional outcomes in various ways, e.g. by facilitating the production of nutritious fruits and plant products, sustaining livelihoods through more efficient production and increasing the diversity of products available in markets. This Guidance presents and expands on six core building blocks for mainstreaming biodiversity for nutrition and health: Cross-sectoral knowledge development and knowledge co-production; Enabling environments; Integration; Conservation and the wider use of biodiversity; Education and awareness-raising; Monitoring and evaluation; This WHO report builds on an unprecedented opportunity to mainstream biodiversity in order to support healthy and sustainable diets, and offers the necessary technical guidance to catalyze and support a transformation of the global food system and transition to healthier, more sustainable diets.