This chapter reports the results of the PM10 and PM2.5 source apportionment at 3 urban background sites (Barcelona, Florence and Milan, BCN-UB, FI-UB, MLN-UB) 1 suburban background site (Athens, ATH-SUB) and 1 traffic site (Porto, POR-TR). Road traffic (sum of vehicle exhaust, non-exhaust and traffic-related secondary nitrate) is the most important source of PM10 (23–38% at all sites) and PM2.5 (22–39%, except for ATH-SUB and BCN-UB). The second most important source of PM10 (20–26%) is secondary sulphate/OC at BCN-UB, FI-UB and ATH-SUB, while it represents 10–14% in MLN-UB and POR-TR. The relative importance of this source is higher in PM2.5 (19–37% at SUB-UB sites). Biomass burning contributions vary widely from 14–24% of PM10 in POR-TR, MLN-UB and FI-UB, 7% in ATH-SUB to <2% in BCN-UB. In PM2.5, BB is the second most important source in MLN-UB (21%) and in POR-TR (18%). This large variability is due to the degree of penetration of biomass for residential heating. Other significant sources are local dust, industries (metallurgy), remaining secondary nitrate (from industries, shipping and power generation), sea salt and Saharan dust. The same analysis is performed for exceedances days. Based on the above, a priority list of measures to improve PM levels is proposed for each city.