Methylglyoxal (MG) is a potent protein glycating agent. Glycation is directed to guanidino groups of arginine residues forming mainly hydroimidazolone N (δ)-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine (MG-H1) residues. MG-H1 formation is damaging to the proteome as modification is often directed to functionally important arginine residues. MG-H1 content of proteins is quantified by stable isotopic dilution analysis tandem mass spectrometry and also by immunoblotting with specific monoclonal antibodies. MG-glycated proteins undergo cellular proteolysis and release MG-H1 free adduct for excretion. MG-H1 residues have been found in proteins of animals, plants, bacteria, fungi and protoctista. MG-H1 is often the major advanced glycation endproduct in proteins of tissues and body fluids, increasing in diabetes and associated vascular complications, renal failure, cirrhosis, Alzheimer's disease, arthritis, Parkinson's disease and ageing. Glyoxalase 1 and aldo-keto reductase 1B1 metabolise >99% MG to innocuous products and thereby protect the proteome, providing an enzymatic defence against MG-mediated glycation. Proteins susceptible to MG modification with related functional impairment are called the "dicarbonyl proteome" (DCP). DCP includes albumin, haemoglobin, transcription factors, mitochondrial proteins, extracellular matrix proteins, lens crystallins and other proteins. DCP component proteins are linked to mitochondrial dysfunction in diabetes and ageing, oxidative stress, dyslipidemia, cell detachment and anoikis and apoptosis. Biochemical and physiological susceptibility of a protein to modification by MG and sensitivity of biochemical pathways and physiological systems to related functional impairment under challenge of physiologically relevant increases in MG exposure are key concepts. Improved understanding of the DCP will likely have profound importance for human health, longevity and treatment of disease.