Several recent studies have reported the costs of adapting to climate change for developing countries and have proved influential in international negotiations given the role of adaptation in a post-2012 climate agreement. However, their estimates range from US$4 to US$109 billion a year. This wide range is symptomatic of the poor state of knowledge. Indeed, compared to the mitigation literature, adaptation cost research is still in its infancy. Important knowledge gaps remain both in terms of scope (whether all impacts are covered) and depth (whether for a given impact all adaptation options are considered). Moreover, the additional costs of adaptation have sometimes been calculated as climate mark-ups against low levels of assumed investments. In many developing countries, low levels of investment have led to a current ‘adaptation deficit’, and this deficit will need to be removed without which the funding for adaptation will be largely insufficient to face the challenge of climate change. Based on a literature review, this article presents the results, underlying hypothesis and main shortcomings of the most recent studies on adaptation costs for developing countries.