Lysosomes are organelles of eukaryotic cells involved in the turnover of various macromolecules. One of their major tasks is the degradation of extracellular material as well as intracellular components that are delivered to lysosomes by endocytosis or autophagy, respectively. Since the products of lysosomal catabolism and certain cytosolic compounds which are destined for degradation have to be transported across the 7-10 nm thick lysosomal membrane it is easy to envisage that this membrane must possess a number of highly specialized proteins. Furthermore, it has to withstand the luminal milieu with an acididc pH of less than 5 and 50 potent hydrolases, maintaining a tight barrier towards the surrounding cytosolic environment. Therefore, lysosomal membrane proteins (LMPs) are usually highly glycosylated probably forming a continuous glycoprotein layer at the luminal side of the lysosomal membrane.