This document provides up-to-date guidance to ASEAN Member States on conducting and using an assessment of the costs of inaction of tackling air pollution based on rigorous scientific methods and best available data. Such assessment would include quantification of pollutant emissions, concentrations, impacts and costs for alternative future scenarios with differential levels of air pollution. It also provides examples of good practices in the ASEAN region which should be built upon throughout the assessment process to ensure the assessment is robust and can be communicated and utilized effectively. Following these guidelines and good practices, ASEAN countries can develop and utilize cost of inaction assessments to overcome current barriers and motivate action on air pollution.