Aging is the outcome of the contest between chemistry and biology in living systems. Chronic, cumulative chemical modifications compromise the structure and function of biomolecules throughout the body. Proteins with long life spans serve as cumulators of exposure to chemical damage, which is detectable in the form of advanced glycation and lipoxidation end products (AGEs, ALEs); amino acids modified by reactive oxygen, chlorine, and nitrogen species; and deamidated and racemized amino acids. Not all of these modifications are oxidative in nature, although oxidative reactions are an important source of age-related damage. Measurements of AGEs and ALEs in proteins are useful for assessing the rate and extent of Maillard reaction damage, but it is the damage to the genome that undoubtedly has the greatest effect on the viability of the organism. The extent of genomic damage represents a balance between the rate of modification and the rate and fidelity of repair. Damage to DNA accumulates not in the form of modified nucleic acids, but as chemically "silent" errors in repair-insertions, deletions, substitutions, transpositions, and inversions in DNA sequences-that affect the expression and structure of proteins. These mutations are random, vary from cell to cell, and are passed forward from one cell generation to another. Although they are not detectable in DNA by conventional analytical techniques, purines and pyrimidines modified by Maillard reaction intermediates may be detectable in urine, and studies on these compounds should provide insight into the role of Maillard reactions of DNA in aging and disease.