Publisher Summary This chapter discusses the higher-order structure of poly(adenosine diphosphate ribose) [poly(ADPR)], poly(ADPR) polymerase purification and its properties, and molecular mechanisms of histone- and elongation-factor (EF-2)-ADP-ribosylation. The precise biological function of the poly(ADP-ribosylation) reaction has not yet been unequivocally established. However, the results are consistent with the occurrence of rapid transient modifications of proteins in localized regions of chromatin. These alterations in chromatin conformation appear to be induced by DNA strand breakage leading to the DNA repair process or to the differentiation process. Thus, after analytical studies in vitro—such as enzyme purification and properties determination and enzyme activity measurements in isolated nuclei—the use of new sensitive analytical techniques in vivo should allow a better understanding of the biological functions of mono and poly(ADPR). The chapter also discusses the process of DNA-repair as one of the biological functions of poly(ADPR) polymerase and presents new techniques for poly(ADPR) studies including immunological analysis.