There are both high resource and political costs in defining and enforcing property rights to water and in managing it with markets. In this paper, I examine these issues in the semi-arid U.S. West where many of the intensifying demand and supply problems regarding fresh water are playing out. I begin by illustrating the current state of water markets in 12 western U.S. states. There are major differences in water prices across uses (agriculture, urban, environmental) and these differences appear to persist, suggesting that water markets have not developed fully enough to narrow the gaps. Moreover, there is considerable difference in the extent and nature of water trading across the western states, suggesting that water values and transaction costs of trade vary considerably across jurisdictions. I then turn to the resource and political costs of defining water rights and expanding the use of markets. In this discussion, efficiency and equity objectives play important, often conflicting, roles. This tension reflects the very social nature of the water resource. To understand the problems of expanding water markets, it is critical to understand the varying political, bureaucratic, and administrative incentives involved.