Main steps are considered of posttranslational modification of lysosomal hydrolases, which are glycoproteins. Processing of the enzymatic carbohydrate moiety in various compartments of endoplasmic reticulum and Golgi apparatus is discussed. Importance of mannose-6-phosphate groups formed during the processing is revealed by studies on binding of these enzymes with specific receptor responsible for their transport into lysosomes. Specificity of lysosomal glycosidases and their isoforms, catalyzing hydrolysis of carbohydrate chains of glycoconjugates and of synthetic substrates dissimilar in the structure, is discussed. Complex structural organization of these enzymes in lysosomes (protein activators and stabilizing factors, presence of marker sites etc) was studied using as an example lysosomal diseases of accumulation, glycosidoses, developed in hereditary deficiency of glycosidases. The data on elevated activity of the majority of lysosomal enzymes in glycosidoses, which are not involved in the primary genetic defect, suggest the possibility of general unspecific response of cells to accumulation of unhydrolyzed compounds.