Marine invertebrates display a tremendous range of thermal sensitivities and thermal tolerance limits, but they can all experience thermal stress at their upper thermal limits. Such limits are shaped by the physical environment inhabited by a species or population and are further influenced by physiological responses to temperature and to thermal changes in macromolecular structure and function. The capacity to compensate for temperature-related changes in metabolism, cardio-respiratory physiology, nerve function, haemolymph oxygen transport, mitochondrial function, protein synthesis, and protective/repair mechanisms are considered and related to spatial variations in thermal tolerance limits in marine invertebrates, such as those observed with latitude and vertical distribution on the shore. Long-term exposure to thermal stress is likely to affect survival as temperatures exceed those required for reproduction and growth, but exposure in combination with alterations in another environmental stressor (e.g. ocean acidification, hypoxia, or salinity) is poorly understood and has produced highly variable results. Greater knowledge of physiological responses across different levels of biological organization, life stages, generations, and spatial scales is required to more fully understand the ability of marine invertebrates to survive warming conditions expected as a result of climate change.