UNEP’s Adaptation Gap Report 2023 has undertaken a comprehensive assessment of the literature and commissioned new studies to provide updated estimates of the cost of adaptation and current adaptation finance flows, and thus the adaptation finance gap for developing countries (the non-Annex I countries defined under the United Nations Framework Convention on Climate Change [UNFCCC]). This background document, the Adaptation Finance Gap Update 2023, provides a more detailed write-up of this analysis.