Methylglyoxal (MG) is a reactive α-dicarbonyl, and may be the most important reactive aldehyde that is primarily generated endogenously in cells and exogenously due to autoxidation process. Interestingly, diabetes was the first disease in which evidence emerged for the increased formation of MG in the body. MG is also a major precursor of advanced glycation end products. It has the ability to enhance protein glycation, oxidative stress, or inflammation, in addition to exerting a toxic effect on insulin secretion from pancreatic β-cells. It is a toxic by-product of glycolysis and many other metabolic pathways. It has been suggested that MG culminates in cytotoxic effects via reactive oxygen and reactive nitrogen species. In mammalian cells, this reactive metabolite is detoxified via the glutathione-dependent glyoxalase pathway forming d-lactate, involving lactoylglutathione lyase (GLO1; EC 4.4.1.5) and hydroxyacylglutathione hydrolase (GLO2; EC 3.2.1.6). Methylglyoxal has been identified as the dominant antibacterial constituent of manuka honey. MG is used in organic synthesis, as a flavoring agent, and in tanning leather. Exposure routes include endogenous formation in the body, ingestion, and inhalation.