Although crustaceans are primarily a marine group, they have diversified into a wide range of environments, including the deep sea, where oxygen and sulphite levels may vary and the littoral zone, where they may encounter daily fluctuations in water levels, temperature and respiratory gas concentrations. Crustaceans have also invaded freshwater and terrestrial habitats, either as facultative air-breathers compensating for the collapse of gills or as fully terrestrial species resisting desiccation. Respiratory gas exchange in water is often limited by hypoxia, both on the seashore in rock pools at night or in the deep sea, particularly around hydrothermal vents and in burrows. Responses vary from behavioral to metabolic via short and long-term physiological adjustments, including changes in the binding properties of respiratory blood pigments. The respiratory requirements of developing eggs are satisfied by active ventilation of the egg mass bythe mother that can determine hatching time. Hypoxia in shallow aquatic environments can be countered by facultative air-breathing, with an initial respiratory and metabolic acidosis countered by mobilization of buffer-base from the exoskeleton, which also serves as a site for sequestration of protons and lactate ions. Temperatures vary both spatially and temporally and can set limits to the distribution of species as they affect metabolic rate, rates of protein synthesis and specific dynamic action following a meal. The ability of crustaceans to compensate for anthropogenic factors such as global warming and acidification of aquatic habitats is poorly understood. Early studies on the effects of ocean acidification imply that survival in adult crustaceans is affected by their ability to compensate for acid-base disturbances. Future studies are likely to concentrate on integrated responses of crustaceans to complex changes in their environment such as the relationship between physiological plasticity and the transcriptome, as well as the ability of crustaceans to undergo genetic adaptations to progressive anthropogenic factors such as those resulting from climate change.