Lysosomal membrane glycoproteins are highly glycosylated proteins decorating the luminal surface of lysosomal membranes. Their biosynthetic route from the rough endoplasmic reticulum to the lysosomal compartment has been elucidated during recent years. Signals for intracellular sorting have been identified and characterized. The function of these proteins remains to be determined. Besides resident proteins the lysosomal membrane harbours at least one transient passenger, lysosomal acid phosphatase, which is sorted as a membrane-bound precursor like resident lysosomal membrane proteins and liberated from the membrane by limited proteolysis upon arrival in dense lysosomes.