Elevations in atmospheric carbon dioxide (CO₂) are anticipated to acidify oceans because of fundamental changes in ocean chemistry created by CO₂ absorption from the atmosphere into the oceans in a process known as ocean acidification. Over the next century, elevated CO₂ is expected to cause a reduction in the pH of the surface ocean from 8.1 to 7.7 units and a reduction in carbonate ion (CO₃^2-) concentration required for calcifying marine organisms. Of growing concern is the potential impact that this change in ocean chemistry will have on marine and estuarine organisms and ecosystems, particularly molluscs and echinoderms which are broadcast spawners with larvae that develop in seawater. Although fertilisation in molluscs and echinoderms appears to be robust to the effects of elevated CO², larval development is characterised by impacts on the rate of larval development through successive stages, larval survival and abnormality including the failure to produce shells and skeletons. Despite these trends, our current understanding of the biological consequences of an acidifying ocean over the next century is still dominated by large uncertainties. Some of the greatest gaps in our understanding is the synergistic impacts of elevated CO₂ with other environmental stressors such as increasing ocean temperature and changing salinity. Until we have a better picture from laboratory and field based experiments which investigate multiple stressors in a chronic way over multiple generations, we will be limited in predicting which mollusc or echinoderm species will be able to acclimate or adapt.