Mucopolysaccharidoses (MPS) are a lysosomal storage disorders caused by deficiency of several enzymes needed for degradation of mucopolysaccharides (glycosaminoglycans). Undegraded glycosaminoglycans accumulate in the cell, part of which are excreted into the urine. There are 10 known enzyme deficiencies that give rise to six distinct MPS. Despite the different enzyme defect, there is clinical similarity between different deficiencies and conversely, wide phenotypic variety among even one enzyme deficiency. Most of the cDNAs corresponding the defect enzyme as well as genomic DNA have been cloned. Mutational analyses of the patient gene have revealed the molecular basis of the disease, correlation of the genotype and phenotype and made the accurate heterozygote diagnosis feasible. Supportive management can greatly improve the quality of life and moreover, development of therapies, such as BMT, administration of recombinant enzyme and gene transfer are in progress for the patients suffering from MPS.