Good mental health is a vital part of people’s well-being, and the COVID-19 pandemic brought renewed attention to its importance. However, discussions so far have not focused sufficiently on how governments should best monitor it at the broader population level, and on how to consider both mental ill-health and positive mental states. This report supports national statistical offices and other data producers in collecting high-quality measures of population mental health outcomes in a more frequent, consistent and internationally harmonised manner. It documents existing measurement practice across OECD countries, discusses the advantages and limitations of available measurement tools, and recommends priority measures to adopt in household, social and health surveys. Measuring Population Mental Health is the first of two reports as part of an assessment of mental health and well-being in the context of the OECD's work on measuring well-being.