The development and application of two numerical models of aerosol transport under urban conditions are discussed. The microscale model is applied to buildings and city canyons of complicated shape using the finite-element method. The mesoscale model is based on finite differences and takes into account the structure of the Earth's surface with the help of the roughness length concept. The results of model calculations for different types of surface inhomogeneities are presented. Qualitative agreement between theory and measurement data is obtained.