Numerous metabolic pathways generate free ADP-ribose at many locations within cells. The metabolic fates of this nucleotide are poorly understood and measurement of it in situ is technically difficult at present. Yet considerable evidence has accumulated implicating that protein glycation by ADP-ribose can occur. This evidence is reviewed here along with recent developments in characterizing the chemistry of this reaction and the application of this information to the identification of this posttranslational modification in protein in situ.