The aim of sustainable farming in both Australia and China is to achieve high water use efficiency (WUE), high profitability and minimal damage to the environment. High WUE is needed to minimise the overuse of scarce water resources. In this chapter, we review the many definitions of WUE that are in use in Australia and China. For example, in rainfed and irrigated zones of southern Australia, WUE or ‘potential yield’ is used as an index of production efficiency and its industry surrogates, whereas in China WUE is estimated as part of water-saving agricultural practices. We also look at why current measures of WUE are not appropriate for some cropping systems in Australia such as those in ‘leaky’ landscapes, where the crops cannot intercept and use all the water that reaches the root zone. A more encompassing measure of plant WUE may need to be developed for these systems and advocated to farmers. To maximise adoption by farmers, WUE must be based on easily measured parameters.