There is global scientific and commercial interest in understanding the impacts of climate warming and heatwaves on the resilience of fish populations. While heat is thought to act at a fundamental level on biological reaction rates, protein structure and cell membrane fluidity, much has been made of the role of oxygen in governing these processes. The “gill-oxygen limitation” (GOL) hypothesis proposes that the growth of fish gills does not keep pace with the increase in metabolic requirements of the body as fish grow, resulting in a progressive mismatch between oxygen supply and demand that is exacerbated at high temperatures. We have empirically tested this hypothesis in thermally acclimated fish in recent years and found no supportive evidence; gills have the plasticity to grow proportionally with metabolism to ensure that oxygen supply and demand are matched as fish grow, even at elevated temperatures. We have, however, found some evidence for oxygen supply limitations across the gills during acute thermal exposures, exacerbated as fish increase in size. This talk will discuss the current state of knowledge on these topics with an aim to shed light on the mechanisms compromising fish resilience to acute heatwaves and chronic warming.