Climate change is increasing the frequency and intensity of extreme weather events like heatwaves. This is of particular concern for ectotherms such as fish as their basic physiological functions are strongly influenced by environmental temperature. It has been suggested that some life stages of fish are much more vulnerable to temperature changes than others. Specifically, some research has predicted that the acute upper thermal tolerance limits of embryos and spawning adults is 10ºC lower than larvae and immature adults. If correct, this would mean that climate change is likely to disproportionately impact embryos and spawners, and these life stages should be brought to the top of global conservation priorities. However, these predictions have been criticised for using vastly different measures of thermal tolerance across life stages, potentially underestimating the thermal tolerance of embryos and spawners. Therefore, a critical question remains: which life stages of fish are most vulnerable to warming? To answer this question, we measured the critical thermal maximum of the above-mentioned four life stages in several fish species from around the world using critical thermal methodology. Results will be discussed in the context of heatwave vulnerability in relation to life history diversity.