Diets are changing everywhere, and the burden of disease associated with unhealthy diets is a worldwide concern. Measurement and monitoring of diets across countries and population groups is critical. However, there are no harmonized metrics for tracking how the healthfulness of diets around the world is evolving. This report assesses the validity, usefulness and fitness for purpose of existing healthy diet metrics as global and national monitoring indicators, presents a comparative assessment of selected healthy diet metrics and discusses priorities and opportunities to improve diet monitoring. This report is an important first step of the Healthy Diets Monitoring Initiative to respond to the need for developing healthy diets metrics for assessing and monitoring diets at national and global level.