Dystroglycan is encoded by a single gene and is cleaved into two proteins, alpha- and beta-dystroglycan, by posttranslational processing. In skeletal muscle, dystroglycan is a component of the dystrophin-glycoprotein complex. alpha-Dystroglycan is an extracellular peripheral membrane glycoprotein that binds to beta-dystroglycan, which is a transmembrane glycoprotein. alpha-Dystroglycan is heavily glycosylated and its sugars have a role in binding to laminin, neurexin and agrin. The main glycan of alpha-dystroglycan was found to be O-mannosylglycan which is a ligand of laminin. We identified and characterized two glycosyltransferases involved in the biosynthesis of O-mannosylglycans. These two enzymes are responsible for congenital muscular dystrophies, muscle-eye-brain disease and Walker-Warburg syndrome. These results indicate that O-mannosylation of alpha-dystroglycan is important in muscle and brain development. This review describes the relation between aberrant glycosylation of alpha-dystroglycan and congenital muscular dystrophies.