Lytic polysaccharide monooxygenases (LPMOs) are a new class of copper-containingmetalloenzymes that have been found to oxidatively degrade polysaccharides (and recently alsooligosaccharides). They dependent on redox partners to provide them with electrons and they utilizemolecular oxygen to cleave their substrate, leaving it oxidized either at C1 and/or C4. They have been foundto act synergistically with hydrolytic enzymes in breaking down biomass, and thus they have great potentialin the production of bioethanol. LPMOs are classified into family AA9-AA11 active on cellulose and chitin(β-1,4 linked sugars) and AA13 active on starch (α-1,4 linked sugars).Here crystallographic studies of LPMOs from the filamentous fungi Lentinus similis (Ls),Chaetomium virescens (Cv), Aspergillus oryzae (Ao) and Thielavia terrestris (Tt) have been carried out.LPMOs belonging to family AA9 and AA13 (LsAA9A and AoAA13) and their interaction with substratehave been structurally characterized. A number of structures of LsAA9A have been obtained in complexwith a range of cellulosic- and hemicellulosic substrates and with the active site Cu in different redox state.Two of the LsAA9A structures with the active site Cu in essentially a Cu(II) state show differences in thenature of the Cu-ligand with and without cellulosic substrate bound and provide structural insight into themechanistic action of LPMOs. Interestingly, for an LsAA9A complex structure with a hemicellulosicsubstrate (xylooligosaccharide) a corresponding difference was not observed. The structure was also solvedfor CvAA9A which shares 46% sequence identity with LsAA9A. A structural comparison of the twoenzymes reveals differences likely to affect substrate binding. Structural studies on AoAA13 did result instarch substrates (maltooligosaccharides) bound in the structure, but failed to provide complex structureswith the substrate bound in the active site. The sites where the bound maltooligosaccharides interacted withAoAA13 were speculated to play a role in substrate recognition. Though most of the AoAA13 structureswhere solved with Zn in the active site, one structure was solved Cu in the active site. Additionally, inAoAA13 structures with less than fully occupied metals in the active site the active site residues where foundto be disordered. Structural studies carried out on TtAA9E with a partially occupied metal site also showeddisorder of active site residues compared to Cu- and Zn-loaded forms of the enzyme.