Glucose, the monomer of cellulose, co-monomer of hemi-cellulose, and one of the components of sucrose, is one of the most important monosaccharides in the acid-catalyzed valorization of carbohydrates. Several pathways have been proposed for its conversion to 5-hydroxymethyl-furfural involving the isomerization of glucose to fructose, the transient formation of 3-deoxy-ᴅ-erythro-hexos-2-ulose or levoglucosenone, and the dehydration at the C-2 position followed by a ring contraction. Isotope labeling experiments have been used to establish the mechanism under specific reaction conditions. Similar mechanisms are operating for the conversion of xylose to furfural.