This paper develops definitions of adaptation and successful adaptation to climate change, with a view to evaluating adaptations. There is little consensus on the definition of adapting to climate change in existing debates or on the criteria by which adaptation actions can be deemed successful or sustainable. In this paper, a variant of the Delphi technique is used to elicit expert opinion on a definition of successful adaptation to climate change. Through an iterative process, expert respondents coalesced around a definition based on risk and vulnerability and agreed that a transparent and acceptable definition should reflect impacts on sustainability. According to the final definition, agreed by the Delphi panel, successful adaptation is any adjustment that reduces the risks associated with climate change, or vulnerability to climate change impacts, to a predetermined level, without compromising economic, social, and environmental sustainability.