The production and consumption of food are responsible for serious environmental degradation. Given the nature of the global economy, the impacts of food are dispersed over the full extent of the planet because food commodities may travel long distances from production to consumption. In this Primer, we introduce the principles of life cycle assessment (LCA), which allows for the assessment of the global extent of the inputs, outputs, and potential environmental impacts throughout the life cycle of a product system. We describe how LCA works following the standard phases of (1) goal and scope, (2) life cycle inventory, (3) life cycle impact assessment, and (4) interpretation. We show that LCA studies can capture the environmental impacts of foods, diets, and food production systems. While LCA has been expanding in scope and breadth, collaboration across disciplines is needed to further capture the diversity of food systems and to better deal with underassessed foods.