Asparagine (N-) linked protein glycosylation is a common and essential post-translational modification of proteins in eukaryotes, archaea and some bacteria. It plays crucial roles in protein folding and in regulation of protein function. Although the general principles of Nglycosylation have been long known, the precise details governing whether a particular asparagine residue will be N-glycosylated or not are not well understood. This is of broad general importance in understanding the structure and function of the immense variety of N-glycoproteins in diverse biological systems. This chapter will review the current understanding of the mechanisms that determine how asparagine residues are selected for glycosylation by the enzyme oligosaccharyltransferase.