N-glycosylation is a highly conserved glycan modification, and more than 7000 
proteins are N-glycosylated in humans. N-glycosylation has many biological 
functions such as protein folding, trafficking, and signal transduction. Thus, 
glycan modification to proteins is profoundly involved in numerous physiological 
and pathological processes. The N-glycan precursor is biosynthesized in the 
endoplasmic reticulum (ER) from dolichol phosphate by sequential enzymatic 
reactions to generate the dolichol-linked oligosaccharide composed of 14 sugar 
residues, Glc3Man9GlcNAc2. The oligosaccharide is then en bloc transferred to 
the consensus sequence N-X-S/T (X represents any amino acid except proline) of 
nascent proteins. Subsequently, the N-glycosylated nascent proteins enter the 
folding step, in which N-glycans contribute largely to attaining the correct 
protein fold by recruiting the lectin-like chaperones, calnexin, and 
calreticulin. Despite the N-glycan-dependent folding process, some glycoproteins 
do not fold correctly, and these misfolded glycoproteins are destined to 
degradation by proteasomes in the cytosol. Properly folded proteins are 
transported to the Golgi, and N-glycans undergo maturation by the sequential 
reactions of glycosidases and glycosyltransferases, generating complex-type 
N-glycans. N-Acetylglucosaminyltransferases (GnT-III, GnT-IV, and GnT-V) produce 
branched N-glycan structures, affording a higher complexity to N-glycans. In 
this chapter, we provide an overview of the biosynthetic pathway of N-glycans in 
the ER and Golgi.