The group of Congenital Disorders of Glycosylation (CDG) is expanding rapidly since the first clinical description of the N-glycosylation defect PMM2-CDG (CDG-Ia) in 1980 (Jaeken et al. 1980). Since then, more than 50 defects have been identified in protein N-glycosylation (some of them also associated with an O-glycosylation defect), in protein O-glycosylation only and in lipid glycosylation (GPI anchor and glycosphingolipid synthesis). Here, we provide a simple approach to the clinical, biochemical and genetic diagnosis of CDG due to a N-glycosylation defect (including combined N-and O-glycosylation defects).