With atmospheric CO2 levels rising unabated, the climate system is warming at the fastest rate in human history. It has long been known that the vast bulk of extra heat associated with anthropogenic carbon dioxide emissions is entering the ocean (e.g., Hansen et al., 1988; Levitus et al., 2000; Barnett et al., 2005), causing warming at the surface and through much of the ocean’s depth. This warming drives the thermal expansion of the ocean that, together with added mass from rapidly melting glaciers and ice sheets, is accelerating the rise in sea levels (Church et al., 2013). As the mean state of the ocean changes, so do its extremes. Marine heatwaves are becoming more frequent and more intense (e.g., Oliver et al., 2018; Hobday et al.), and increasing sea levels are resulting in more damage from storms in coastal regions (e.g., Hauer, 2017). In addition, marine species are being significantly affected, with populations moving poleward to escape the heat (e.g., Sorte et al., 2010), loss of key habitat-forming species (e.g., Wernberg, 2013, 2016), and mass bleaching and mortality of coral reefs (e.g., Hughes, 2017). Simultaneously, more intense hurricanes, destructive wildfires, and severe droughts have been occurring across the global terrestrial landscape. With these ocean and land changes as a backdrop, this special issue of Oceanography examines some of the new science around ocean warming that involves the use of ocean observations and modeling, and that range from changes in the physical environment to impacts on marine ecosystems.