Diseases of glycosylation, usually referred to as congenital disorders of glycosylation (CDG), are rare inherited disorders ascribing defects of nucleotide-sugar biosynthesis and transport, glycosyl transfer events and vesicular transport. Most CDGs cause neurological impairment ranging from severe psychomotor retardation to mild intellectual disability. Defects in N-glycosylation are the main cause of CDGs (Marquardt & Denecke 2003, Grunewald et al. 2002, Hennet 2012, Goreta et al. 2012) and can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Jaeken et al. 1984, Stibler & Jaeken 1990). Disorders of O-glycosylation, glycosaminoglycan and glycolipid metabolism have recently been discovered and, together with N-glycosylation, represent the major pathways affected by glycan biosynthetic disorders (Freeze 2006, Jaeken 2011). In addition, glycosylation diseases associated with the enzymes that mediate the biosynthesis of glycosylation precursors are described in this section. As the number of these disorders has increased, nomenclature has been simplified so that now, the name of the mutant gene is followed by the abbreviation CDG (Jaeken et al. 2009). Effective therapies for most types of CDGs are so far not available (Thiel & Korner 2013).