Water markets in Australia's Murray-Darling Basin (MDB) and the western USA are compared in terms of their ability to allocate scarce water resources. The study finds that the gains from trade in the MDB are worth hundreds of millions of dollars per year (note that all monetary units of dollars in this article are treated as US$ because Australian$ are converted at par). Total market turnover in water rights exceeds US$2 billion per year while the volume of trade exceeds over 20% of surface water extractions. In Arizona, California, Colorado, Nevada and Texas, trades of committed water annually range between 5 and 15% of total state freshwater diversions with over US$4.3 billion (2008 US$; monetary units in dollars are expressed in their value in US$ in 2008) spent or committed by urban buyers between 1987 and 2008. The two-market comparison suggests that policy attention should be directed towards ways of promoting water trade while simultaneously mitigating the legitimate third party concerns about how and where water is used, especially in conflicts between consumptive and in situ uses of water. The study finds that institutional innovation is feasible in both countries and that further understanding about the size, duration and distribution of third party effects from water trade and how these effects might be regulated, can improve water markets' ability to manage water scarcity better.