A wide range of strategies may be considered for the synthesis of oligosaccharides in vitro using enzymes, all of which present significant challenges to the enzyme technologist. Many simple oligosaccharides may be produced by the hydrolysis of readily-available polysaccharides using specific enzymes. However, to produce the complex branched hetero-oligosaccharides of the types which occur N-linked to glycoproteins is more taxing. Materials of this type may be synthesised using the natural synthetic enzymes which employ sugar nucleotides as substrates. These enzymes are highly specific but they are costly to use due to their instability and to the cost of their substrates. It has been demonstrated that glycosidases are capable of synthesising hetero-oligosaccharides when provided with underivatised sugars in conditions of low water activity but that the specificity of synthetic reactions is apparently not high and that yields of material are low. Approaches to these problems are discussed, including the use of immobilised enzymes in packed-bed reactors to allow the ’ping’ stage of the synthetic reaction to be separated in time from the ’pong’ stage, and the application of aqueous two-phase systems which may be ’tailored’ to separate the enzyme and the substrates from the final product. The ability to synthesise a range of oligosaccharides is dependent on the availability of appropriate glycosidases with differing specificities. There is a clear importance of ’biodiversity’ in providing knowledge of sources of these.