Given the importance of the availability of valid information on population exposure to air pollutants, the WHO European Centre for Environment and Health organized a working group to define the features of monitoring networks that allow their use in assessing the potential exposure of the population to air pollution from ambient air. The research resulted in this volume. The principles outlined are intended to promote progressive modification of the networks monitoring air quality to improve their usefulness for health impact assessment. The book is directed specifically at network managers, to those who design new networks or modify existing ones, to policy-makers and to those who influence policy.