Parasitic helminths share many interesting features in their energy metabolism, although they are not one homogeneous group of organisms; there are large differences among them in, for example, structure and physiology. At some stage of their development they all have an outstanding capacity for anaerobic metabolism, while in other stages an aerobic metabolism prevails. Parasites have to adapt to different environments during their life cycle. These variations in external conditions strongly influence their lipid and energy metabolism. Here an introduction is presented to the current ideas on the bioenergetics and lipid metabolism of parasitic helminths, focusing on the differences between various stages.