The need to identify an instream flow requirement for water resources management exists in a number of countries. The use of habitat simulation in the United States, Great Britain, and France to assist in finding the requirement is reviewed. The habitat simulation techniques considered are based on the logic of the Physical Habitat Simulation System (PHABSIM) of the U.S. Fish and Wildlife Service. All three countries are doing work to understand the link between habitat and aquatic populations. In the United States, this work is focused toward an improved understanding of the time series aspects of habitat analysis; in France, toward improved understanding of the structure of habitat in a stream; and in England and Wales, toward improved modelling of the habitat.