New concerns about crop coexistence in agricultural landscapes are being expressed in reaction to the prospect of introducing transgenic crops into European cropping systems: these include meeting current consumer demand for non-GM products, respecting threshold levels required for organic farming labels as well as keeping food cultures separated from those destined for the pharmaceutical and energy industries. To address these concerns in the case of maize crops, we have chosen a modelling approach. Our aim was to simulate cross-pollination in the case of existing agricultural landscapes, taking into account the effect of climate and cropping techniques in order to forecast gene escape from genetically modified maize to non-GM maize. The resulting spatially explicit model, MAPOD (Matricial Approach to POllen Dispersal), is presented in this paper. A preliminary evaluation is also provided. Pollen exchanges between GM and non-GM maize crops are simulated and influencing factors such as field sizes and shapes, distribution of GM and non-GM fields in the agricultural landscape as well as flowering dates and dynamics are integrated. Model parameter values were either derived from existing models of pollen dispersal or estimated from experimental field studies. The preliminary evaluation of MAPOD was carried out by comparing simulation results with data from two French and one American gene flow field trials. MAPOD was found to provide good average predictive values. Examples of output data illustrate the capacity of the model to simulate a wide range of agricultural contexts. These simulation results provide a basis for designing coexistence rules and monitoring procedure set-up.