The present invention relates to the enzymatic synthesis of oligosaccharides, including sialylated product saccharides. In particular, it relates to the use of recombinant cells to take up low cost precursors such as glucose, pyruvate and N-actyl-glucosamine, and to synthesize activated sugar moieties that are used in oligosaccharide synthesis. The methods make possible the synthesis of many oligosaccharides using microorganisms and readily available, relatively inexpensive starting materials.