It is argued in this paper that current attempts at community involvement in planning at the watershed level have not been fully realised. This is demonstrated through implementation research, reported here, of community involvement in urban stormwater planning in over twenty watersheds in Australia. It is concluded that narrowly operationalised technical expertise; implementing officers' values towards communities' capacity to understand complex problems; and the basic technical planning rationale are structural impediments to effective community involvement. Effective public participation in this field facilitates the re-definition of the causes and effects of urban stormwater pollution. Fundamental to the realisation of effective public participation is the recognition of the importance of, and role-played by different forms of knowledge (such as scientific, technical, political, social, local, indigenous, bureaucratic and historical) in this process. For improved policy implementation, the key is recognising that problem definition frames the understanding of stormwater pollution causes and effects and pre-determines the expertise requirements for subsequent planning processes. Effective public participation can result in a shared conception of the issues of stormwater conceptually lifting the problem definition out of the narrowly defined technical arena to a social phenomenon reflecting community concerns and aspirations.