Gangliosides are a large group of complex lipids found predominantly on the outer layer of the plasma membranes of cells, and they are particularly concentrated in nerve endings. Their half-life in the nervous system is short, and their membrane composition and content are strictly connected to their metabolism. Their neobiosynthesis starts in the endoplasmic reticulum and is completed in the Golgi; catabolism occurs primarily in the lysosomes. However, the final content of gangliosides in the plasma membrane is affected by other cellular processes.In this chapter structural changes in the oligosaccharide chains of gangliosides induced by the activity of glycohydrolases and in some cases by glycosyltransferases that are associated with plasma membranes are discussed. Some of the plasma membrane enzymes arise from fusion processes between intracellular fractions and the plasma membrane; however, other plasma membrane enzymes display a structure different from that of the intracellular enzymes. Several of these plasma membrane enzymes have been characterized and some of them seem to have a specific role in the nervous system.