It is well established that in Europe, high O 3 concentrations are most pronounced in southern/Mediterranean countries due to the more favorable climatological conditions for its formation. However, the contribution of the different sources of precursors to O 3 formation within each country relative to the imported (regional and hemispheric) O 3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O 3 Target Value set by the European Air Quality Directive. O 3 source attribution is a challenge because the concentration at each location and time results not only from local biogenic and anthropogenic precursors, but also from the transport of O 3 and precursors from neighbouring regions, O 3 regional and hemispheric transport and stratospheric O 3 injections. Our study applies and thoroughly evaluates a countrywide O 3 source apportionment method implemented in a Chemical Transport Model (CTM) at high resolution (4 × 4 km) over the Iberian Peninsula (IP) to understand and quantify the origin of peak O 3 events over a 10-day period covering the most frequent synoptic summer conditions in the IP. The method tags both O 3 and its gas precursor emissions from source sectors within one simulation and each tagged species is subject to the typical physical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NO x local sources to high O 3 concentrations compared to the contribution of imported O 3 . We show for the first time that imported O 3 is the largest input to the ground-level O 3 concentration in the IP, accounting for 46 % to 68 % of the daily mean O 3 concentration during exceedances of the European Target Value. The hourly imported O 3 increases during typical northwestern advections (70–90 %, 60–80 µg/m 3 ), and decreases during typical stagnant conditions (30–40 %, 30–60 µg/m 3 ) due to the local NO titration effect. During stagnant conditions, the anthropogenic precursors control the O 3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O 3 concentrations are strongly affected by vertical mixing of O 3 -rich layers present in the free troposphere, which result from local/regional layering and accumulation, and continental/hemispheric transport. Indeed, vertical mixing largely explains the presence of imported O 3 at ground level in the Iberian Peninsula. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O 3 concentrations for local O 3 management, being the O 3 source apportionment an essential analysis prior to the design of O 3 mitigation plans in any non-attainment area. To achieve the European O 3 objectives in southern Europe, ad hoc local actions should be complemented by decided national and European-wide strategies.