Flash flooding in urban drainage basins is controlled by the interplay of a wide range of hydrologic, hydraulic and hydrometeorological processes. From the hydrometeorological perspective, the distribution of extreme rainfall rates at ``short" time scales and ``small" spatial scales is of fundamental importance for flash flooding. Measurement of extreme rainfall rates by weather radar at the appropriate time and space scales is the cornerstone of flash flood forecasting. The particular technical issues that arise in addressing this problem will be examined. Hydrologic modeling for flash flood forecasting is complicated in urban drainage basins by alterations to the drainage network, especially through the storm drain system of an urban drainage basin, and alterations to the infiltration properties of a basin, especially through detention basins and impervious cover. Representation of the dominant hydrologic processes controlling flash flood response is a major challenge for advances in hydrologic modeling in urban drainage basins. The hydraulic properties of urban stream channels are profoundly influenced by bridges, channelized reaches, channel stabilization projects and development of floodplains. Hydraulic properties of an urban stream channel are also altered by the river itself as it adapts to changing hydrologic response of a drainage basin. These alterations to the hydraulic properties of urban stream channels play an important role in determining the flood hazards of an urban drainage basin. Enhanced hydraulic modeling capabilities are needed for determining the propagation of flood waves through urban stream channels and the extent of inundation for a given discharge. Examples are drawn from a number of urban drainage basins in the United States, with special focus on the Baltimore metropolitan region.