Congenital disorders of glycosylation are group of hereditary resulting in severe psychomotor retardation and multiorgan failure. So far eleven different defects were identified on the pathway of N-glycan biosynthesis. Seven of them belong to CDG type I and result in incomplete occupation of potential N-glycosylation sites. Four defects were found in N-glycan processing. Molecular background of CDG and potential perspectives of therapy are summarised and updated in this review.