Current stormwater quality control pond design in Ontario typically includes the specification of a uniform detention time for extended detention ponds to ensure water quality control. In reality, the pollution-control performance of such facilities is governed by two opposing forces: improved pollutant removal efficiency over longer detention times and the decreased volume of runoff captured and treated by the facility for longer detention times. The combination of these effects produces a maximum attainable level of long-term pollution-control performance at an optimal detention time. Derived probability distribution models for runoff control are used to investigate the quality-control behaviour of extended detention dry ponds for the case where a design storm is specified to establish pond storage requirements and for the more general case where storage requirements may be selected on the basis of the level of control desired. Graphical relationships are developed to facilitate the planning and design process. The methodology presented encourages efficient engineering design while promoting environmental protection by ensuring a maximum level of long-term pollution control.Key words: stormwater management, water quality, probabilistic models, optimization, stormwater ponds, detention time, pond design.