A detailed sensitivity analysis of ozone concentrations with respect to anthropogenic and biogenic emissions is performed at European scale in summer 2001 through the use of the chemistry-transport model Polair3D. We estimate the time evolution of the sensitivities and the extent of the sensitive regions. We discriminate the chemical species to which photochemistry is the most sensitive. This work is intended as a preliminary study for inverse modeling of emissions. Local sensitivities are computed using a tangent linear model and an adjoint model of the underlying chemistry-transport model. Global sensitivities are approximated by means of Monte Carlo simulations. It is shown that NO emissions have a prominent impact and that VOC emissions also play an important role. Major emission sources are associated with the highest sensitivities, although a non-negligible sensitivity of the concentrations at observation stations can cover the whole domain. A typical relative sensitivity of ozone concentrations to NO emissions is about 6 ¿g ¿ m-3, which is low as to compared to the error and the uncertainty in output concentrations.