Caused by synoptic changes and the diurnal cycle, the atmospheric boundary layer is never steady state. The unsteadiness is especially pronounced within and above the urban canopy layer. In former times, it was not possible to take the natural variability of the urban atmosphere properly into account, due to a lack of both computer power for models of adequate sophistication and sufficiently matured measurement techniques. Instead of this, quasi-steady situations were assumed, despite the fact that in reality they do not exist. The situation has improved now. After a brief description of the numerical tools which are presently available, their potential to simulate urban flow and dispersion episodes is assessed. The importance of validating these tools is stressed, and the question of how to obtain reliable validation data is discussed. Using combinations of field and laboratory data for the validation procedure is recommended. Finally, at the concrete example of puff dispersion within the urban canopy layer, it is demonstrated how such data sets can be generated and actually applied.