Climate change is resulting in unprecedented increases in both mean and extreme global temperatures, with widespread effects on Earth’s biota. It is difficult to predict the impacts of climate change, and resulting extreme heat events, on species with complex life cycles. Throughout their complex life cycles, life stages of marine invertebrates may experience unequal levels of environmental stress (exposure) and have differing thermal tolerances (sensitivity), which vary in space and time. A meta-analysis of differences in sensitivity across life stages of marine invertebrates revealed that, when exposed to the same levels of warming, younger life stages are more sensitive than older life stages (Chapter 1). We used field observations of environmental conditions experienced coupled with laboratory assessments of thermal tolerance to investigate whether thermal safety margins differed across three benthic life stages of the California mussel, Mytilus californianus. Our results demonstrated that thermal sensitivity decreased throughout ontogeny, and, since exposure trends were similar across life stages, sensitivity drove thermal safety margin differences across life stages and approaches (Chapter 2). Finally, we investigated how sensitivity and exposure vary across small-scale habitats in space and over four seasons for one year, calculating mortality risk for two benthic life stages of mussels. We deployed temperature loggers to document habitat-specific exposure patterns, conducted lab thermal tolerance assays to calculate sensitivity, and used field surveys to determine whether risk patterns were reflected in abundance distributions. We found that sensitivity was more variable across seasons (time) than small-scale habitats (space), resulting in high temporal variation in mortality risk (Chapter 3). Together, these results highlight the importance of incorporating life stage and seasonal dynamics when predicting impacts of climate change on the survival of marine invertebrates.