1. This paper introduces, and summarises the key messages of, a series of papers that emanated from a symposium on the Role of Drought in the Ecology of Aquatic Systems, held in Australia in 2001.2. Defining drought hydrologically is problematic because the return times, intensity, duration and long-term trends in low-flow periods are specific to regions and times. Droughts may instead be referred to as 'significant low-flow periods', many of which have been replaced by 'anti-drought' conditions in rivers as they are used increasingly as irrigation conduits.3. Droughts can be divided into those that cause predictable, seasonal press disturbances and less predictable, protracted 'ramp' disturbances. However, while droughts may be 'ramp' disturbances, their effects on aquatic biota are most likely to be 'stepped' when geomorphological or hydrological thresholds are crossed, causing abrupt changes in biological community structure and ecosystem processes.4. Physical, morphological, physiological or behavioural refugia confer resistance or resilience to riverine populations and communities that experience drought conditions. The physical and chemical parameters associated with refugia habitats and their formation, influence population parameters within, and interactions among, species and can have protracted reproductive consequences, even well after the cessation of the drought.5. Fish, invertebrate and plant populations and assemblages seem to recover rapidly from drought. Most studies of the effects of drought, however, have arisen fortuitously and have involved relatively short temporal, and small spatial, scales. Innovative approaches, such as microsatellite DNA analyses, can reveal that the effects of drought may be profound and long-lasting, resulting in population bottlenecks and altering the course of the evolution of species.6. During periods of drought, decreases in inputs of dissolved organic carbon, nitrogen and phosphorus may lead to carbon limitation to microbial metabolism, resulting in autotrophic production being favoured over heterotrophic production.7. Long-term climate trends, as indicated by palaeoecological evidence, suggest that, at least for Australia, droughts are likely to occur more frequently in the future. Anthropogenic effects on climate are likely to exacerbate this.8. It is important that drought is seen for what it is: a natural extreme of the flow continuum, with flooding at the other extreme. Thus, despite the potential for dramatic impacts on aquatic biota and the negative social connotations associated with such events, drought must be incorporated into river management plans.