A lthough the common foundations of site-scale ecosystem restoration are well understood, the spatial scale and duration of restoration are rapidly expanding, raising theoretical questions and practical concerns. For instance, the primary goal of the Bonn Challenge, issued jointly in 2011 by the International Union for Conservation of Nature and the Government of Germany, is to restore 350 million ha of degraded land by 2030, while the UN General Assembly recently proclaimed 2021-2030 to be the Decade on Ecosystem Restoration. Such coordinated restoration across large spatial and temporal scales is a response to widespread environmental degradation, human welfare needs, and increased understanding of how species are sustained by distributed habitats and ecosystems (Lotze et al. 2006; Hall et al. 2018). In view of these trends, the Society for Ecological Restoration (SER) recently formed a Large-Scale Ecosystem Restoration section (Daoust et al. 2014). What does ecological restoration science offer to those working toward such ambitious goals? Restoration ecology provides information about the study of individual sites, ecosystems, and vulnerable species developed over the past half century (Roman and Burdick 2012; Clewell and Aronson 2013), yet for the most part it has not addressed large-scale restoration that includes multiple ecosystems and restoration projects across landscapes. Large-scale restoration is usually more cost-effective than local site-specific planning (Neeson et al. 2015); however, little formal research on achieving successful program-level outcomes has been reported. Useful principles to support the enormous projected expansion of restoration and ensure that large investments produce planned ecosystem functions are urgently needed. In practice, large-scale restoration is typically overseen by multidisciplinary teams and based on an ecosystem approach developed at the site scale, as can be seen in the programs we reviewed (Figure 1). Geomorphic conditions and hydrological