Marine heatwaves (MHWs), discrete periods of anomalously warm seawater, have become longer, more frequent and more intense over the past century, driven largely by anthropogenic warming. Their impacts span biological scales, from physiological stress and mass mortalities to range shifts and the loss of habitat-forming foundation species such as kelps, seagrasses and corals, with effects typically most severe at the warm edges of species' ranges. These ecological changes cascade into society, disrupting fisheries, aquaculture, tourism and cultural practices, with economic losses from individual events reaching hundreds of millions to billions of US dollars. In this seminar I describe what we know about the biological and socioeconomic impacts of MHWs, and the steps being taken towards preparedness and intervention. A strong El Niño is forecast for 2026/27 and is expected to drive potentially record-breaking MHW activity. What impacts can we expect, what monitoring should we be starting now, and what interventions should we consider? I will discuss these questions and introduce a global protocol designed to capture subtidal ecological responses before, during and after the event.