Do ecosystems regenerate? Issues of complexity and scale, both spatial and temporal, lie at the heart of analyses of ecosystem regeneration. To what extent is it possible to test hypotheses of ecosystem regeneration? Related concepts of reforestation, restoration, and rewilding warrant attention. As in other levels of organization, ecosystem regeneration necessitates consideration of whether renewal of form or renewal of function (or both) are required. To get at some of these questions, we consider shifting baselines and historical ecology, especially as applied to marine ecosystem restoration.