This paper puts forward the case for more widespread evaluation of the health impacts of WASH interventions. It argues that more frequent evaluation would encourage investors and implementers to focus on impacts rather than outputs, and would enable more objective comparative assessment of the value-for-money of different types of urban WASH intervention. Importantly the paper notes that health impact evaluation need not be as costly as is widely thought, and proposes that the before-after concurrent control (BAC) design may be the most appropriate method in urban WASH evaluation contexts.