Book excerpt: Ensuring optimal diets and nutrition for the global population is a grand challenge fraught with many contentious issues. To achieve food security for all and protect health, we need functional, equitable, and sustainable food systems. Food systems are highly complex networks of individuals and institutions that depend on governance and policy leadership. This book explains how interconnected food systems and policies affect diets and nutrition in high-, middle-, and low-income countries. In tandem with food policy, food systems determine the availability, affordability, and nutritional quality of the food supply, which influences the diets that people are willing and able to consume. Readers will become familiar with both domestic and international food policy processes and actors, and they will be able to critically analyze and debate how policy and science affect diet and nutrition outcomes.