Tropospheric photooxidant pollution was investigated in detail for the first time over the Paris area during the Air Pollution Over the Paris Region (ESQUIF) project. From 1998 to 2000, 12 intensive observation periods (IOPs) were carried out. They represented various meteorological situations, all leading to strong polluted events over Paris and its surroundings. During these periods, measurements were performed with a new strategy of circular flights around the city, coupled to stations or remote sensing surface measurements. Such data obtained at various altitudes and at different ranges from the city center document the evolution of pollution events on horizontal and vertical scales. In addition, ESQUIF also allowed for the evaluation of models developed in parallel to the project. In this overview, ESQUIF is presented in terms of the set of IOPs. Periods are compared in terms of meteorology and resulting types of pollution episodes. The occurrence of these latter events is discussed in terms of local production and influence of long-range transport. Using both measurements and model simulations, some important results are highlighted, especially concerning accuracy of boundary conditions, processes of mixing within the boundary layer, surface emissions estimation (including biogenic), and photolysis attenuation. Finally, results from data assimilation studies and sensitivity studies using adjoint modeling and a Monte Carlo approach are also presented.