In 2006, the World Health Organization (WHO) published an update of its Air Quality Guidelines, originally written in 1987 and last updated in 2000. In this issue, Michal Krzyzanowski, who led the updating for WHO, and Aaron Cohen provide an overview of the guidelines, which can be applied throughout the world. Two commentaries accompany the review by Krzyzanowski and Cohen. John Bachmann, who was involved in the development of air quality standards in the USA for decades, offers his views on the utility of global guidelines. Kirk Smith, who has focused his research on biomass fuels and indoor air pollution, comments on the place of guidelines directed at outdoor air quality in advancing public health.