This chapter provides a review of the fascinating genetic disorders known as mucopolysaccharidoses that are caused by the intralysosomal accumulation of glycosaminoglycans in various tissues. Due to absent activity of a variety of lysosomal enzymes, a characteristic clinical phenotype usually develops in early childhood and then evolves with coarse facial features, thick skin, corneal clouding as the direct expression of excessive storage, mental retardation, growth deficiency and skeletal dysplasia as manifestations of defective cellular function. Bone abnormalities referred to as dysostosis multiplex are also characteristic and often dramatic. The classical disorders are Hurler and Hunter disease which are attributable to deficiencies of α-L-Iduronidase and Iduronate-2-sulfatase, respectively. The genes causing mucopolysaccharidoses have been cloned and numerous mutations identified. Thus, the diagnoses of mucopolysaccharidoses can be readily accomplished by recognition of the clinical phenotype combined with biochemical and molecular studies. Treatments are challenging, although enzyme replacement strategies and stem cell transplantation show some promise.