Containing papers presented at the twenty-first in a successful series of conferences on the modelling, monitoring and management of air pollution, the book Air Pollution XXI covers what has become a widespread and growing challenge to the international community.  Governments face a need to balance concern over its known impacts on local and global health and the environment with improving or maintaining economic development.  The key to achieving that balance is to use science to identify the nature and scale of air pollution impacts and to formulate effective policies and regulations.  As our knowledge and application of the science of air pollution improves, we are better able to predict, assess and mitigate the implications air pollution has for local, regional, national and international economic systems.