In this chapter, we explore how concerns about regional air quality began with the discovery of the effects of acid rain, where environmental damage in one country was due to air pollution transported from other countries. These concerns led to the development of regional air quality models. The important regional air pollutant, ozone, is then examined and how its reduction is necessary to improve urban and regional air quality and ease the build-up of global warming. Emissions inventories are examined and how they can be used to assist environmental bodies and governments by providing input data for air quality models and formulate policies to improve future air quality. The role of air quality legislation is explored and how international bodies can work together to reduce regional air pollution.