Worldwide there is growing awareness of the pivotal role of the flow regime (hydrology) as a key 'driver' of the ecology of rivers and their associated floodplain wetlands. Ecological processes related to flow and other factors govern the ecosystem goods and services that rivers provide to humans, such as flood attenuation, water purification, production of fish and other foods and marketable goods. Protecting and restoring river flow regimes and hence the ecosystems they support by providing environmental flows has become a major aspect of river basin management. Over 200 approaches for determining environmental flows now exist and they are used or proposed for use in more than 50 countries worldwide. Most methodologies currently used in Australia and southern Africa and increasingly in other countries, are holistic in their scope, recognising that it is necessary to provide water for aquatic ecosystems from source to sea and for all water-dependent ecological components. This paper provides a brief history of the development of environmental flow methods and identifies the main features and strengths of each, giving most emphasis to holistic or ecosystem methodologies. We then present an overview of research initiatives needed to enhance these approaches and improve their capacity to predict the ecological, social and economic consequences of change in river flow regimes.