Several recent studies have reported adaptation costs for climate change, including for developing countries. They have similar-sized estimates and have been influential in discussions on this issue. However, the studies have a number of deficiencies which need to be transparent and addressed more systematically in the future. A re-assessment of the UNFCCC estimates for 2030 suggests that they are likely to be substantial under-estimates. The purpose of this report is to illustrate the uncertainties in these estimates rather than to develop new cost estimates, which is a much larger task than can be accomplished here. The main reasons for under-estimation are that: (i) some sectors have not been included in an assessment of cost (e.g. ecosystems, energy, manufacturing, retailing, and tourism); (ii) some of those sectors which have been included have been only partially covered; and (iii) the additional costs of adaptation have sometimes been calculated as ‘climate mark-ups' against low levels of assumed investment. In some parts of the world low levels of investment have led to a current adaptation deficit, and this deficit will need to be made good by full funding of development, without which the funding for adaptation will be insufficient. Residual damages also need to be evaluated and reported because not all damages can be avoided due to technical and economic constraints. There is an urgent need for more detailed assessments of these costs, including case studies of costs of adaptation in specific places and sectors.