Publisher Summary This chapter describes the cubic phases of lipids. Phases with cubic symmetry have been observed in lipid-water systems since the early days of lipid polymorphism, at a time when it seemed all too natural to presume without closer inspection that phases with such high symmetry should consist of spherical micelles of type I or 11, orderly packed in a cubic lattice. Another important aspect of the cubic phases of lipid-containing systems is their possible biological relevance. In a more general way, the biological significance of lipid polymorphism is a problem often evoked in the past. On account of their remarkable topological properties, the bicontinuous cubic phases are far better candidates for biological speculations than the lamellar and the hexagonal phases.