This chapter discusses ADP-ribosylation, which is a novel type of posttranslational covalent modification of proteins using NAD as a donor of the modification group. ADP-ribosylation reactions are classified into two major groups: mono- and poly(ADP-ribosyl)ation. Both types are distributed widely among eukaryotes and prokaryotes. Among a number of nuclear proteins known to serve as acceptors of poly(ADP-ribosyl)ation, histones have been studied most extensively in vitro as and in vivo . All of five major subfractions of histone—HI, H2A, H2B, H3, and H4—are ADP-ribosylated, although to different degrees. Degradation of poly(ADP-ribosyl) protein is carried out by three enzymes. Poly(ADP-ribose) glycohydrolase hydrolyzes the ribosyl- ribose bond inside the polymer to yield ADP-ribose; phosphodiesterase in mammalian cell nuclei hydrolyzes the pyrophosphate bond in the polymer and monomer of ADP-ribose to yield iso-ADPribose and AMP; and ADP-ribosyl protein lyase, which was formerly termed ADP-ribosyl histone splitting enzyme or, erroneously, ADP-ribosyl histone hydrolase, splits the linkage between ADP-ribose and protein. Evidence has been accumulating that suggests roles of poly(ADP-ribosyl)ation of nuclear proteins in DNA repair, cell differentiation, and oncogenesis. Condensation or decondensation of chromatin induced by poly(ADP-ribosyl)ation of histones and modifications of some enzyme activities by poly(ADP-ribosyl)ation or poly(ADP-ribose) have been postulated as the underlying mechanisms of these functions.