Information on the cost of adaptation in freshwater systems is necessary to better design strategies to face climate change and water management. We look at the existing estimates with the aim of identifying research gaps. Our analysis shows that case study-specific literature is scarce, fragmented, and not always methodologically transparent. At the same time, most existing global assessments are likely to represent underestimates and rely heavily on each other. We conclude that a clear conceptual framework is still missing. Remaining research gaps include addressing inter-sector linkages and estimations of other than only direct costs, in addition to addressing the issues of ‘adaptation deficit’ and ‘residual damage’.