The oomycete phylum comprises devastating crop pathogens that represent a serious threat to food production and sustainability. Their cell walls share structural features with both plants and fungi. Like plant cells, oomycete hyphae contain cellulose as the main load-bearing component, whereas chitin, a typical major cell wall component of fungi, occurs in minute amounts in the walls of some oomycete species only. Similar to fungal cell walls, oomycete produce a diversity of β-glucans that consist essentially of β-(1,3) and β-(1,6) glucosidic linkages. Thus, oomycetes represent interesting comparative model systems for cellulose biosynthesis in plants and β-glucan and chitin biosynthesis in fungi. In addition, the enzymes responsible for cell wall biosynthesis in oomycetes represent potential targets of inhibitors that can be used to control the diseases provoked by pathogenic species. However, the proteins associated to the oomycete carbohydrate synthase complexes and their corresponding mechanisms are not well characterised. In this lecture I will present our latest results on the characterisation of the cell wall glycome of ten different pathogenic oomycetes, highlighting species-specific structural epitopes. I will also describe the structural and biochemical properties of the vital membrane-bound glycosyltransferases that form cellulose and chitin in two devastating pathogens, Phytophthora capsici, which infects a large number of crops, and the fish parasite Saprolegnia parasitica. Different recombinant forms of the enzymes were expressed in heterologous systems and characterised in vitro providing insight into the molecular mechanisms and structural organisation of chitin and cellulose synthases.