The transport and dispersion of pollutants in a street canyon, and the intersection between two streets, have been studied using wind-tunnel experiments and numerical simulations. The study of the street canyon demonstrates the importance of the geometry of the canyon (aspect ratio, asymmetry) in determining both the topology of the flow and the concentration distribution; the flow is also very sensitive to wind direction. The study of the street intersection shows how the intersection influences the flow and dispersion in the adjoining streets. This work has been used to develop new and practical models for flow and dispersion in city streets; these models are compared here with the results from wind-tunnel experiments and numerical simulations.