AbstractPoly(ADP‐ribose) and poly(ADP‐ribose) polymerase (PARP) were discovered about 40 years ago, but their significance was not well elucidated until recently. In the early stage of the history of PARP, the presence of antibodies in the sera of human patients with lupus erythematosus indicated its natural occurrence. PARP, as well as the degrading enzyme, poly(ADP‐ribose) glycohydrolase (PARG), are present in most eukaryotes except for yeasts. Studies that used inhibitors of PARP indicated the involvement of PARP and poly(ADP‐ribose) in DNA damage repair, and eventually PARP was purified and the gene was cloned. Molecular analysis then revealed various functional domains, such as the one for binding to strand breaks of DNA. Parp‐1–deficient and Parg‐deficient cells showed, in general, enhanced sensitivity to the lethal effects of ionizing radiation and alkylating agents. Parp‐1 knockout mouse embryonic stem cells developed into teratocarcinoma‐like tumors when injected subcutaneously into nude mice, these tumors featuring giant cells similar to syncytiotrophoblastic giant cells with hyperploidy. Parp‐1 was also found in centrosomes, suggesting that poly(ADP‐ribose) and PARP‐1 are functionally involved in the maintenance of chromatin structure and the equal distribution of chromosomes into daughter cells. Intriguing findings on the real biological significance continue to be generated, with new light shed on mechanisms of carcinogenesis and pointing to novel cancer treatments. Highlights during the last four decades of studies by laboratories focusing on poly(ADP‐ribose)/PARP, including our own, are condensed and summarized in this review. © 2003 Wiley‐Liss, Inc.