Air quality assessment is frequently driven by the need to determine whether a standard or guideline has been exceeded. This overshadows another objective of air quality assessment: providing the information needed to estimate population exposure to air pollution and the effects on the health of the population. Consequently, most existing air quality monitoring systems do not fully address population exposure to toxic air pollution. Given the importance of these data for air quality management, this report describes strategies and methods for providing information on ambient air quality that is adequate for health impact assessment. Human exposure to air pollution may result in a variety of health effects, depending on the types of pollutants, the magnitude, duration and frequency of exposure and the associated toxicity of the pollutants of concern. People are exposed to air pollutants both indoors and outdoors depending on the activities of individuals. It is important to assess the exposure levels of different population groups, especially sensitive or susceptible individuals such as children, elderly people and chronically ill people. Health impact assessment combines estimates of population exposure with information on toxicity or the relationship between exposure and response. Information on the relationship between exposure and response is necessary to estimate the potential health risks. The estimates of health effects for a population base are typically calculated in terms of predicted excess negative health effects (such as increases in hospital admissions or mortality) caused by exposure to a certain level of air pollution. This involves combining the information on the response to certain concentrations derived from epidemiological or toxicological studies with the number of people exposed to each concentration of air pollution in the community being assessed The report discusses the overall requirements for designing and operating networks for monitoring ambient air and a range of air quality models used in comprehensive programmes for air quality assessment and management. The report does not detail monitoring practice and methods. However, it is intended to prompt good practice in monitoring and health impact assessment. To this end, several important issues are identified and recommendations are made on each step of the process of monitoring air quality. Specific details of these issues are discussed in the body of this report and in the other reports cited in it. The following points are addressed to decision-makers and designers of systems for air quality assessment.