Never before in recorded human history have there been as many extreme climatic events as in the past decade, and anthropogenic climate change is now recognised as a major contributor to this trend. Droughts, floods, cyclones, cold snaps and heat waves are all linked through Earth's climate systems and can have significant ecological and socio-economic impacts on land and in the oceans. Despite growing appreciation of the importance of discrete, extreme events in determining ecosystem structure the vast majority of knowledge stems from terrestrial research, even though marine ecosystems play a central role culturally, socially and economically in the lives of most people. Marine ecosystems provide a myriad of ecological goods and services, including nutrient cycling, food and other resources, biogenic coastal defence and climate regulation, all of which have substantial socioeconomic value. Coral reefs, seagrass meadows and temperate kelp forests are particularly valuable in terms of capital generated from recreation, fishing activities, coastal defence and biodiversity, and contribute trillions of pounds to the global economy each year. In the UK alone, the estimated direct economic value of marine biodiversity exceeds £20 billion per year. In marine environmental research, much attention has been given to ocean acidification and, more recently, plastic pollution, yet there is a strong argument to suggest that extreme warming events (i.e. 'marine heatwaves' (MHWs)) pose an even greater risk to ecosystems. In the past decade alone, MHWs have devastated entire ecosystems and severely affected fisheries, aquaculture, food webs and carbon cycling. The frequency and duration of MHWs has increased significantly in recent decades and is predicted to increase throughout the 21st Century, as a consequence of anthropogenic climate change. Despite the unequivocal importance of MHWs in structuring ecosystems, our current understanding of their impacts remains poor. Knowledge of responses to MHWs stems from only a few events, such as the 1998 El Niño episode, the Mediterranean MHW of 2003 and the 2011 warming event off Western Australia. The 2011 MHW off Western Australia, for example, resulted in major shifts in benthic ecosystem structure in a tropical-temperate transition zone, by causing widespread mortality of cool-water habitat forming species. This project will address critical knowledge gaps in marine climate change ecology. It will synthesise existing information on ecological responses to MHWs and use a novel analytical approach to conduct a global-scale analysis of their impacts. The project will also carry out a range of experiments and surveys to examine how key organisms and processes are affected by MHWs with differing physical attributes. Finally, predictions of future patterns and impacts of MHWs will be made, based on physical and ecological modelling techniques. This project will significantly advance understanding of the impacts of extreme climatic events in the global ocean and will be of direct relevant to climate change mitigation and adaptation, as society must safeguard valuable coastal marine ecosystems against increased climatic stress in the coming decades.