In this chapter, an overview is given of flow over uniformly rough boundaries, with special applications to urban areas. We distinguish two different types of boundary layer flows typical for different meteorological situations, and with different applications in urban wind studies. The first is high velocity boundary layer flow, which is neutrally stratified, and which can be modelled in a conventional boundary layer windtunnel. For the rough surfaces of urban areas, this is a very typical flow condition of broad applicability, in particular if one also consideres the transitional flow from one roughness to another - for which a simple model is described in an appendix. The second is low velocity flow, typical for air pollution situations. It often is also neutrally stratified, but the most extreme pollution cases occur for stratified flows with a lower mixed layer and an elevated inversion. In later chapters, the effect of stratification on boundary layer flows in urban areas and its influence on wind engineering problems are described in more detail.