Sphingolipidoses is characterized by the accumulation of excessive quantities of a related family of complex lipids in various tissues of the body. The nature of the metabolic defect is demonstrated in four sphingolipodystrophies. This chapter summarizes the metabolic defects in these sphingolipodystrophies—namely, in Gaucher's disease, meta-chromatic leukodystrophy, Fabry's disease, and Niemann–Pick disease. The lipids that accumulate in all of the sphingolipidoses have a portion of their structure in common, called ceramide, which consists of the long-chain amino alcohol sphingosine and a long-chain fatty acid bound through an amide bond to the nitrogen atom of sphingosine. Various substituents are linked to carbon-I of the sphingosine portion of ceramide, such as glucose in glucocerebroside, which accumulates in Gaucher's disease and phosphorylcholine in sphingomyelin, which accumulates in Niemann–Pick disease. The use of labeled materials for the elucidation of the defects in Tay–Sachs disease and related conditions is also indicated.