Lytic polysaccharide monooxygenases (LPMOs) are copperdependent enzymes that catalyse the oxidative degradation of several polysaccharides, such as cellulose, chitin, starch, and pectin. LPMOs work by activating (otherwise inactive) C-H bonds in the glycosidic bonds that link the monomers in the respective polysaccharides. This activation ultimately leads to oxidation and cleavage of the glycosidic bond. Several mechanistic pathways have been suggested for LPMOs, and we have summarized the most recent suggestions in Figure . Originally, LPMOs were discovered in bacteria and fungi. Today, they have been found across all kingdoms of life (though not yet in mammals) and exhibit a growing number of functions. For instance, LPMOs and LPMO-like proteins were recently suggested to also play crucial roles in plant, insect and human pathogenesis such as blood diseases and meningitis. However, the function that has so far attracted the most attention is a significant boosting effect on biomass degradation. Nowadays, LPMOs are employed in industrial enzyme cocktails to produce biofuels and other value-added products. To date, eight different LPMO families have been discovered, namely AA9-AA17 (AA12 does not contain LPMOs). All members share a common active site: a copper centre coordinated by two histidine residues in a motif termed the histidine brace (see Figure ). Initially, LPMOs were thought to be strictly monooxygenases, catalysing the reaction