The international field experiment Pianura Padana Produzione di Ozono (PIPAPO) studying the volatile organic compounds (VOC)/NOx ozone production sensitivity took place in May and June 1998 downwind of the metropolitan area of Milan. The project was embedded in the framework of the EUROTRAC-2 Limitation of Oxidant Production (LOOP) subproject. Several ground stations between the city of Milan and the Alps north of Milan combined with airborne measurements delivered a comprehensive data set suitable for observation-based analysis and validation of chemical transport models. The special section devoted to the PIPAPO project contains 10 contributions on the characterization of secondary aerosol formation, and radical precursors, and the characterization of ozone production sensitivity based on field measurements and numerical model simulations. In this paper the scientific background and the major objectives of PIPAPO are described. An overview of the field measurement program, the study site, and the meteorological conditions prevailing during the experiment are given. A general conclusion of the combined results is that under typical summer conditions in the Milan area with clear skies, low wind speeds and high temperatures, the transition from VOC- to NOx-sensitive ozone production occurs a few tens of kilometers downwind of the strongest emission sources in the city of Milan.