The sensitivity of marine crustaceans to ocean acidification is poorly understood, but can be assessed by combining data from physiological and ecological studies. The species most at risk are exclusively marine and have limited physiological capacities to adjust to environmental change. They are poor iono- and osmoregulators and have limited abilities to compensate for acid–base disturbances. The problems are compounded in slow-moving, relatively inactive species because they have low circulating protein levels and low buffering capacities. Species living in low-energy environments, such as deep-sea and polar habitats, are particularly vulnerable, because they are metabolically limited with respect to environmental change. Elevated pCO2 levels in seawater, such as those predicted for the year 2300, are known to have diverse effects on calcification rate, little effect on egg production and a negative effect on growth rate and moulting frequency in marine crustacean species. At these levels, embryonic development is negatively impacted, but larval and juvenile stages do not appear to be affected, unless the changes in pCO2 are accompanied by rising temperatures. Overall, marine crustaceans are broadly tolerant to the seawater pCO2 levels expected by 2100 and 2300, but only in the medium-term (weeks) and only in the more adaptable species. The reductions in growth rate are of concern, as these changes could affect species survival, distribution and abundance. Studies are urgently needed to evaluate whether the patterns of vulnerability identified here in crustaceans will still be relevant after long-term (months) exposure to the relevant pCO2 levels, in combination with changes in other environmental factors.