Congenital (genetic) disorders of glycosylation (CDG) are a rapidly growing disease family, with some 45 members reported since its first clinical description in 1980. Most of these are protein hypoglycosylation diseases, but recently three defects in lipid glycosylation have been identified. Most protein hypoglycosylation diseases are due to defects in the N-glycosylation pathway (16 diseases). The remaining ones affect the O-glycosylation pathway (8 diseases), both the N- and the O-glycosylation pathways, or other pathways (17 diseases). CDG can affect nearly all organs and systems, but there is often an important neurological component. The first-line screening for the N-glycosylation diseases is serum transferrin isoelectrofocusing (IEF), and for the O-glycosylation disorders apo CIII IEF. It has to be stressed that a normal test result does by no means exclude a CDG. In case of an abnormal result and as long as the basic defect has not been elucidated, the disease is labeled CDG-x (CDG-Ix when the transferrin IEF shows a type 1 pattern, and CDG-IIx when it shows a type 2 pattern).