Embedding ambient air quality standards (AAQS) in legislation is an important foundation for effective national air quality governance. Legislation can establish institutional responsibility for air quality standards, set mechanisms for accountability, public participation and enforcement, and institutionalize processes for setting and updating robust air quality standards as knowledge and technologies develop. UNEP’s 2021 Regulating Air Quality: The First Global Assessment of Air Pollution Legislation found that, while most countries in the world do embed AAQS in legislation, there was no common legal framework for AAQS globally. UNEP has developed this Guide on Ambient Air Quality Legislation to assist national lawmakers and policymakers to develop or improve ambient air quality legislation. The aim of this Guide is to promote robust national systems of air quality governance that prioritize public health outcomes and respect that all humans share the same need to breathe air of adequate quality. The Guide emphasizes that air pollution is a collective problem arising from decisions and behaviours across a wide range of policy sectors. Thus, regulatory alignment across wide-ranging policy areas is critical to achieving AAQS in practice. This Guide addresses a lacuna that exists in air quality laws globally, providing a legal resource for developing robust national legislation that supports public access to scientifically evidenced levels of clean air.