Poly (ADP-ribose) polymerase (PARP) is a kind of enzymes transferring ADP-ribose units from NAD+ onto adjacent nuclear proteins, thus forming long branched poly ADP-ribose (PAR) chains that recruit other repairing enzymes to the site of damage. PARPs function as key signal in DNA excision and repair, apoptosis, and chromatin structure and participate in transcriptional regulation of several signaling pathways, including genes involved in inflammation. Some PARP proteins are capable of activating Wnt signaling and driving tumor cell proliferation. Herein, PARPs have become important cancer drug targets. PARP inhibition has been found to avoid tissue damage in diseased animal models and also causes PARPs to be trapped on the damaged DNA to generate toxic PARP-DNA complexes that prevent cancer cells from proliferating and replicating. This emphasizes the significance of developing PARP inhibitors that show dramatic therapeutic potential for breast and ovarian cancer. PARP inhibitors have also been investigated as combination therapies with immune checkpoint inhibitors.