Publisher Summary Carbohydrate-deficient glycoprotein (CDG) syndromes are a new group of diseases first reported in 1980. They are genetic multisystemic diseases characterized by the presence of defective carbohydrate moieties on secretory, lysosomal and probably membranous glycoproteins and possibly also on glycolipids. In all reported patients to date, the nervous system was moderately to severely affected and most other organs were involved to a variable degree. This chapter discusses the clinical biochemistry of CDG. Three types of CDG syndrome have been described in the literature to date. These syndromes are characterized by different carbohydrate defects in glycoproteins. Serum transferrin is the glycoprotein that is generally used as a marker to identify the nature of the carbohydrate defect. In particular, by immune isoelectrofocusing, the pattern of the transferrin glycovariants in the three types of syndrome is quite different and may be used as a first step in the differential diagnosis. However, this method that is only based on the number of negative charges is not without pitfalls. In fact, transferrin has to be fully iron saturated before electrophoresis to avoid additional separations due to the Fe(III) charges.