Transboundary water issues affect nearly everyone. There are 276 international river basins and the United Nations has defined nearly 300 transboundary aquifers. The basin areas that contribute to these rivers comprise approximately 47 percent of the land surface of the Earth, include 40 percent of the world’s population, and contribute almost 60 percent of the world’s freshwater flow. Water sources for 800 million people living in 39 countries originate beyond their national borders. Within each international basin and overlying each transboundary aquifer, the demands from environmental, domestic, economic users, and the inputs of pollution increase annually, while the amount of freshwater in the world remains roughly the same as it has throughout history. Given the scope of the problems and limited resources available to address them, water wars are inevitable. Or are they? Surface water and groundwater crossing international boundaries present increasing challenges to regional stability because hydrologic needs can often be overwhelmed by political considerations. Yet wars are expensive, disruptive, and interfere with the efforts to relieve human suffering, reduce environmental degradation, and achieve economic growth.