This entry reviews atmospheric processes affecting pollutant transport and diffusion over complex terrain, focusing in particular on the peculiarities of processes over mountains when compared to flat terrain. In fact, pollutant dispersion processes over complex terrain are much more complicated than over flat areas, as they are affected by atmospheric interactions with the orography at different spatial scales. In particular, atmospheric flows over complex terrain are characterized by a continuous and interacting range of scales, from synoptic forcing to mesoscale circulations and turbulence fluctuations. In complex terrain, the mechanical and thermal influence of the orography can modify the large-scale flow and produce smaller-scale motions which would not exist on flat terrain, thus enhancing the spatial and temporal variability of atmospheric processes relevant for pollutant dispersion.