This report describes a unique collaboration among investigators from Europe, the United States, and Canada using existing data from three geographic areas and supported by HEI in collaboration with the European Commission. APHENA offered a large and diverse data set with which to address methodological as well as scientific issues about the relationships between PM10, ozone, and mortality and morbidity that were the subject of lively debates at the time the project was launched. Drs. Katsouyanni and Samet and their colleagues undertook a rigorous examination of time-series methods used to model the relationship between daily PM10 and ozone concentrations and daily mortality and hospital admissions. They sought to develop a standardized approach to the analysis of time series data at the city and regional level, to assess the consistency between relative rates of mortality and hospital admissions across Europe and North America when estimated using a common analytic protocol, and to explore possible explanations for any remaining variation in the results that analytic differences could not explain.