Ocean acidification caused by the increased uptake of atmospheric carbon dioxide by the oceans is likely to have serious impacts on marine organisms that make shells and exoskeletons from calcium carbonate. The consequences for the provision of marine ecosystem services such as fisheries and services generated by coral reefs are uncertain but potentially severe. In this chapter we set out a framework for the economic assessment of impacts from ocean acidification. We review the existing economic literature on ocean acidification, which is nascent and sparse. To date only a partial set of the potentially impacted ecosystem services have been assessed with a focus on the direct use values that can be more easily addressed. Gaps in the current knowledge are identified and avenues for future research are discussed. Comparing the existing impact estimates for ocean acidification with those for climate change show them to be an order of magnitude lower. Due to the relatively proximate impacts of ocean acidification, however, the implications for optimal mitigation of carbon dioxide emissions may be substantial.