The attachment of N-glycans to nascent glycoproteins in the endoplasmic reticulum (ER) is intimately related to glycoprotein biogenesis. Processing of N-linked oligosaccharides begins in the ER and participates in glycoprotein folding and assembly. The elucidation of N-glycan processing mechanisms in the ER is uncovering their role in glycoprotein biosynthesis.