Abstract During ten-plus years of debate over the proposed Cape Wind facility off Cape Cod, Massachusetts, the public's understanding of its environmental, economic, and visual impacts matured. Tradeoffs also have become apparent to scientists and decision-makers during two environmental impact statement reviews and other stakeholder processes. Our research aims to show how residents' opinions changed during the debate over this first-of-its-kind project in relation to understandings of project impacts. Our methods included an examination of public opinion polls and the refereed literature that traces public attitudes and knowledge about Cape Wind. Next we conducted expert elicitations to compare trends with the level of understanding held by small groups of scientists and Cape Cod stakeholders. Our review found that Massachusetts residents became more supportive of the project while our research demonstrated the gap between scientific and lay knowledge diminished late in the debate. To facilitate planning for other offshore energy projects, we recommend steps to move the public to an informed position more quickly.