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The energy metabolism of the liver fluke, Fasciola hepatica, offers good examples of adaptations in metabolism in parasitic helminths. Adaptations in the metabolism of parasitic helminths are twofold. On the one hand, adaptations occur to the fact that for the parasitic stages, the environment (host) provides many substrates that can be used directly for anabolic as well as catabolic purposes. Therefore, parasitic stages have lost (parts of) many of the anabolic pathways common to non-parasitic organisms. On the other hand, parasitic helminths continuously adapt their metabolism to the different environments they encounter during their life cycle, while alternating between free-living and parasitic stages. In this review the carbohydrate metabolism of F. hepatica will be discussed as model system for these adaptations. Adaptations to the parasitic way of life, as well as to the changing environment will be presented. Special attention will be paid to transitions in carbohydrate metabolism during the development in the final host, from an aerobic energy metabolism in the juvenile liver fluke to an anaerobic one in the mature adult. Changes in the electron-transport chain, including the quinone used in the different types of metabolism, will also be discussed.