Abstract: Asparagine linked glycosylation is one of the most common protein modification reactions in eukaryotic cells, as many proteins that are translocated across or integrated into the rough endoplasmic reticulum carry N-linked oligosaccharides. Although the primary focus of this review will be the structure and function of the eukaryotic oligosaccharyltransferase, key findings provided by the analysis of the archaebacterial and eubacterial oligosaccharyltransferase homologues will be reviewed, particularly those that provide insight into the recognition of donor and acceptor substrates. Selection of the fully assembled donor substrate will be considered in the context of the family of human diseases known as Congenital Disorders of Glycosylation. The yeast and vertebrate oligosaccharyltransferase are surprisingly complex hetero-oligomeric proteins consisting of seven or eight subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Stt3p, Wbp1p and Swp1p in yeast; ribophorin I, DAD1, N33/IAP, OST4, STT3A/STT3B, Ost48 and ribophorin II in mammals). Recent findings from several laboratories have provided overwhelming evidence that the STT3 subunit it critical for catalytic activity. Here we will consider the evolution and assembly of the eukaryotic oligosaccharyltransferase in the light of recent genomic evidence concerning the subunit composition of the enzyme in diverse eukaryotes.