Increasing numbers of people live in flood-prone areas worldwide. With continued development, flooding will become more frequent. Acceleration of the hydrological cycle and sea-level rise resulting from climate change could worsen the problem. We must therefore address the social, economic and political factors that force or allow some people to inhabit high-risk areas. For example, monsoon flooding last year of Thailand’s Chao Phraya River caused damage to Bangkok and surrounding areas estimated at US$45 billion. A failure to prepare for this recurrent hazard, which has occurred in each of the past four decades, is partly to blame. Short-term engineering approaches are not enough. Building higher dykes or cascades of dual-purpose dams may maximize water storage and reduce flood risk, but they can make people complacent and thus more vulnerable to floods. Long-term development solutions are needed. Vulnerable cities need to be redesigned, for example by supplying transport links to metropolitan areas on higher ground.