The mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders caused by a deficiency of lysosomal enzymes catalyzing the degradation of glycosaminoglycans (GAG) (mucopolysaccharides). Depending on the enzyme deficiency, the catabolism of dermatan sulfate, heparan sulfate, keratan sulfate, chondroitin sulfate, or hyaluronan, may be blocked singly or in combination. Lysosomal accumulation of GAG molecules results in cell, tissue, and organ dysfunction. The MPSs share many clinical features, although in variable degrees. The disorders are chronic and progressive, and usually display a wide spectrum of clinical severity within one enzyme deficiency. GAG fragments generated by alternative pathways are excreted in urine, providing the basis for diagnostic screening for the MPS.