As the prospects of significantly mitigating climate change through emissions reductions become dimmer, the critical necessity of adaptation has become clearer, with failure-to-adapt litigation possibly playing an important role in bringing adaptation measures to pass. Based on a review of every adaptation-related case in the U.S. Climate Litigation Database maintained by the Sabin Center for Climate Change Law, this Article offers the first comprehensive assessment of failure-to-adapt litigation in the United States. It finds that such cases have proliferated in this country over the past decade, but that the lawsuits so far filed have sought specific, incremental, and relatively small-scale adaptation measures rather than systemic, large-scale, coordinated action. The Article’s central finding is that failure-to-adapt litigation in the United States has so far been only modestly successful: most suits have failed, but a significant minority have succeeded. Failure-to-adapt litigation succeeds frequently enough to make it an important, and perhaps underutilized, tool for bringing about much-needed adaptive measures in the United States.