The paper presents an action theory to analyse adaptation to climate change. It provides hypotheses on barriers to adaptation and a framework to analyse them. It clarifies what is meant by „adaptation measures“ and critically reflects on established concepts. Currently, many observers indicate substantial barriers to cope with the impacts of climate change. However, the often incoherent use of terms like autonomous and planned adaptation seems to be of little help here. There are no established methods that enable for operationalizing research on human or social adaptation. The action theory of adaptation intends to allow for rigorously clarifying the notion of adaptation in each specific research context, and delivers a template for hypothesizing on adaptation. The argument is developed by framing adaptation as an individual or collective action, and by building on established analyses of (social) action. To represent the bio-physical counterpart of adaptation, concepts from the DPSIR framework are used. The exposition of the concepts is underpinned by case examples. We conclude that basic components of the theory are (collective) actors, means and ends of adaptation. Ends may be targeted at socio-economic or bio-physical units that are exposed to climate change, but also at other receptors. The theory highlights that climate change affects many actors in different ways, and that their reactions are strongly interlinked. Actions tend to come in means-ends-chains. For understanding adaptation, it is necessary to address these interlinkages. Important barriers are caused by (1) a mismatch of the means that are necessary for an adaptation, that are available, and that are actually employed; (2) externalities and high transaction costs due to the interlinked actors, receptors and units that are exposed to climate change.