The abundant nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1), represents an important novel target in cancer therapy. PARP-1 is essential to the repair of DNA single-strand breaks via the base excision repair pathway. Inhibitors of PARP-1 have been shown to enhance the cytotoxic effects of ionizing radiation and DNA damaging chemotherapy agents, such as the methylating agents and topoisomerase I inhibitors. There are currently at least five PARP inhibitors in clinical trial development. Recent in vitro and in vivo evidence suggests that PARP inhibitors could be used not only as chemo/radiotherapy sensitizers, but as single agents to selectively kill cancers defective in DNA repair, specifically cancers with mutations in the breast cancer associated (BRCA) 1 and 2 genes. This theory of selectively exploiting cells defective in one DNA repair pathway by inhibiting another is a major breakthrough in the treatment of cancer. BRCA1/2 mutations are responsible for the majority of genetic breast/ovarian cancers, known as the hereditary breast ovarian cancer syndrome. This review summarizes the preclinical and clinical evidence for the potential of PARP inhibitors in genetic breast and ovarian cancers.