Genomic instability is a hallmark of tumourigenesis, influencing tumour development and progression. In particular, defects in the DNA damage response (DDR) have been extensively investigated and are known to shape therapeutic response. Since immune checkpoint blockade (ICB) therapy has been approved for treatment of tumours with defective mismatch repair the interplay between DDR pathway deficiency and the immune system has been of particular interest. The cGAS/STING signalling pathway has recently emerged as a key mediator of inflammation in response to DNA damage.This was identified through transcriptional profiling of BRCA1/2 deficient breast cancers and Fanconi Anaemia (FA) patient bone marrow, revealing a common transcriptional subgroup associated with BRCA1/2 and FA deficiency characterised by upregulation of innate immune signalling genes. Additionally, it is now apparent that the DNA damage arising from a multitude of DNA repair defects and DNA damage induced by some classical chemotherapies/radiation also has the ability to induce an innate immune response mediated by cGAS/STING activation. Here we review the role of intrinsic and extrinsic DNA damage in mediating immune activation and its context within tumourigenesis, as well as the potential therapeutic opportunities it represents for the treatment of cancer, such as combining DNA damaging agents with immunotherapies. •The cGAS/STING driven innate immune response can be driven by DNA repair deficiencies such as BRCA1/2.•The cGAS/STING pathway can be driven by chemo/radio-therapy induced DNA damage.•Activation of the cGAS/STING pathway enhances anti-cancer immune checkpoint blockade therapy.