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Tumor survival, genomic stability, and therapy resistance are dictated by the DNA damage response (DDR). Although poly (ADP-ribose) polymerase (PARP) inhibitors have established the DDR as a therapeutic target, many tumors evade first-generation drugs by rewiring their adaptive repair pathways and imposing microenvironmental constraints. This review synthesizes recent discoveries in key DDR pathways, such as PARP, ataxia telangiectasia and Rad3-related kinase (ATR), ataxia telangiectasia mutated kinase (ATM), checkpoint kinase 1 (CHK1), WEE1 G2 checkpoint kinase (WEE1), and DNA-dependent protein kinase (DNA-PK), and describes the next-generation inhibitors designed to increase selectivity and circumvent resistance. We also analyze the role of hypoxia, stromal remodeling, inflammatory cytokines, and immune-cell plasticity in the tumor microenvironment in determining DDR dependency and response. Special attention is paid to cGAS-STING, immunogenic signaling via damage-associated molecular patterns (DAMPs), and mechanisms that convert a cold tumor into a hot one. Lastly, we touch upon the new nanocarrier-based delivery approaches that enhance pharmacokinetics, target resistant tumor niches, and expand the possibilities for combinatorics with immunotherapy and radiotherapy. Collectively, these findings provide a guide to the implementation of next-generation DDR inhibitors and nanomedicines to deliver a more accurate, durable, and context-specific cancer therapy.