A pronounced seasonal variability in primary productivity is the crucial factor in determining the life strategies of many polar zooplankton species, whereas oligotrophic tropical oceans experience little change with a low productivity. Lipid accumulation represents an important energetic adaptation in pelagic organisms to cope with the pronounced seasonal productivity in polar oceans. We compare important global euphausiid species from the Arctic via the tropics to the Antarctic, focussing on the genera Euphausia and Thysanoessa. While the Antarctic krill Euphausia superba is the species with the highest biomass of all metazoans on earth, krill species are clearly less relevant in other oceans. In polar krill species total lipid accumulation is usually very pronounced and may be utilised for metabolic maintenance during overwintering or for reproductive processes in spring. Most polar krill species store large amounts of the unusual depot lipid phosphatidylcholine (lecithin), a polar lipid with highly unsaturated fatty acids. However, they exhibit strong differences in their neutral lipid compounds, which may either consist of wax esters or triacylglycerols, but also of both types of lipid classes. Their major end-products of the fatty acid and fatty alcohol biosynthesis are quite different and usually species-specific. In contrast, tropical euphausiids do not rely on lipids as energy reserves and exhibit the usual lipid and fatty acid compositions of biomembranes with high amounts of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). In polar euphausiids, the major end-products of the fatty acid and fatty alcohol biosynthesis are quite different and usually species-specific. The ecophysiological implications of these deviating lipid characteristics may determine biogeographical zonation patterns and affect the vulnerability of polar krill species to global warming.