Integrated water resources optimization models (IWROM) are tools that have been developed over the last decade for determining optimal water allocations among competing sectors. This article describes the state of the art of IWROMs. We illustrate the various approaches that have been taken to determine and maximize economic benefits of withdrawing water for various use categories in IWROM applications, including off-stream human uses and in-stream uses such as ecological flows. First, we describe the hydrologic simulators used in IWROM applications, and the mathematical methods used to solve the optimization problems. It is suggested that IWROMs (a) seek to model coupled human–nature relationships and mimic the impact of water resources management strategies on the environment at the basin scale; (b) allow for the simulation and assessment of economic policies and strategies on water resources management; (c) can support basin-wide decision-making; and (d) are particularly useful for water-scarce regions. Finally, we have identify the need for improvements in (a) simulating biophysical systems; (b) handling model uncertainty; (c) inclusion of environmental flows and other relevant environmental factors through economic benefit functions; (d) accounting for social impacts related to shifts in water allocations among users; and (e) inclusion of stakeholders in the development of IWROMs.