Enzyme-catalysed glycosylation offers an alternative to chemical methods. The natural choice for this are glycosyltransferases (GTs) that are well-represented in a variety of families in the CAZy database () , with each family potentially harbouring numerous specificities . Nevertheless, GTs have proven to be rather difficult to handle in vitro and often require expensive nucleotide-glycoside donors . Therefore, glycoside hydrolases (GHs) offer an alternative for glycosynthesis. These enzymes are particularly abundant and their diversity in terms of bond breaking ability matches carbohydrate complexity itself. GHs catalyse hydrolysis using a variety of mechanisms, but the majority perform two subsequent displacements at the anomeric carbon, thus yielding a product whose anomeric configuration is identical to that of the substrate . GHs operating by such a mechanism are termed "retaining" GHs and represent 67% of all GHs (755496 classified modules) in the CAZy database, grouped in 83 out of its current 161 GH families (as of February 24 th , 2020). Allowing for rare exceptions , all other GHs are inverting. A result of the double displacement mechanism is that retaining GHs possess the intrinsic potential to catalyse transglycosylation, thus to synthesize glycosidic bonds. Even though most retaining GHs have strong hydrolytic activity and weak, often undetectable transglycosylation activity, some display significant levels of transglycosylation. This reaction is under kinetic control and modulated by a number of factors related to the reaction conditions , including acceptor concentration, water activity, substrate activation, temperature, pH, but particularly enzyme properties. Accordingly, engineering of GHs has proven to be a potent way to obtain transglycosylases .