The intracellular degradation of glycoproteins occurs predominantly in the lysosomes through the concerted action of proteases and glycosidases. Genetic defects in any of the enzymes cleaving the oligosaccharide side chains lead to specific diseases because of an excessive lysosomal accumulation of partially degraded material, mostly oligosaccharides. This paper presents an overview of the biochemistry and the clinical spectrum of this group of diseases including sialidosis, galactosialidosis, alpha- and beta-mannosidosis, fucosidosis, aspartylglucosaminuria, and alpha-N-acetylgalactosaminidase deficiency (Schindler disease). In addition, the sialic acid storage disorder (Salla disease) which is caused by a defect in the lysosomal transport of this acidic monosaccharide is included because of functional and clinical correlations.