Integrated Water Resources Management has been identified as one of the basic water resources related policy approaches in several recent important commitments and recommendations including the Johannesburg Summit and World Water Forums. Recognizing that IWRM is largely still a theoretical concept with not much sound scientific research, our objective is to investigate how the IWRM process in the Mekong Basin is institutionalised, and implemented in practice. We focus on the Cambodia's Great Lake, the Tonle Sap. Of particular interest are the bottlenecks of IWRM, both those that are within the water sector and those that relate to various, more general development tendencies.