Knowledge of the number density and size distribution of atmospheric aerosols in the submicron size range is essential for estimation of the aerosol radiative effects and the health impact of particles. Ultrafine particles (UFP), i.e. particles with diameters <100 nm, have received increased attention in recent years with improved possibilities of measuring them. UFP originate from nucleation, involving gas-to-particle conversion, and direct emission from primary combustion processes. As ultrafine particles contribute negligibly to particulate matter mass they should be described in terms of particle number (PN) concentrations. Given the potential health effects due to PN exposure, it is important to obtain PN concentrations with high spatio-temporal resolution. One of the major difficulties in providing accurate estimates of PN is the complexity of atmospheric processes affecting aerosol evolution, e.g. coagulation, deposition, and photochemical formation. Recently, we started the development of the EMEP/MSCWmodel towards simulating size-resolved particle number and mass over Europe. Aerosol dynamics in the EMEP model are calculated with the sectional multicomponent aerosol module MAFOR. The performance of MAFOR has been tested extensively against smog chamber experiments and observation data from field campaigns in remote marine and urban environments. The aerosol dynamic processes included are particle nucleation (activation or kinetic parameterisations), coagulation by Brownian motion of particles, and growth due to condensation of sulphuric acid and low volatile/semi-volatile organic vapours (presently only from biogenic VOCs).