This chapter provides an update on the adaptation finance gap for developing countries (defined as the non-Annex I countries under the United Nations Framework Convention on Climate Change [UNFCCC]), as reported in previous Adaptation Gap Reports (AGRs) (UNEP 2014; UNEP 2016a; UNEP 2016b; UNEP 2018; UNEP 2021). It has reviewed the evidence base on the estimated costs of adaptation, including recent studies, and also considered the emerging estimates of country adaptation needs from National Adaptation Plans (NAPs) and Nationally Determined Contributions (NDCs). This provides an updated view on the potential costs of adaptation. It has also reviewed the latest data on global adaptation finance flows. This allows, in theory, a comparison of finance flows against the estimated adaptation costs, and thus makes it possible to determine the potential size of the adaptation finance gap (and whether this is changing) in developing countries. However, the analysis of both adaptation costs and finance flows is very challenging (UNEP 2016a; UNEP 2021). In this respect, this chapter provides insights rather than new numbers. Finally, it provides an update on the opportunities and progress to bridge the gap and discusses new insights since the 2020 edition of the AGR (UNEP 2021).