Ongoing drought and a rapidly growing population in the naturally dry Southwest have created a high demand for water, a limited resource. With this demand comes a need to understand not just the science of water, but also the economics. Water has traditionally been treated as a public good, but in areas where it is increasingly scarce, it is becoming a tradable commodity through the development of water markets. In this issue's feature, we look at when, where, and why water may be a commodity, particularly in the Southwest, and how water markets work.