cGAS surveillance of micronuclei links genome instability to innate immunity DNA is strictly compartmentalized within the nucleus to prevent autoimmunity. Despite this, cGAS, a cytosolic sensor of dsDNA, is activated in autoinflammatory disorders and by DNA damage. The mechanism by which cellular DNA gains access to the cytoplasm remains unclear. This study shows that cGAS localizes to micronuclei arising from genome instability in a model of monogenic autoinflammation, after exogenous DNA damage and spontaneously in human cancer cells. These micronuclei, which occur after mis-segregation of DNA during cell division, consist of chromatin surrounded by their own nuclear membrane. Breakdown of the micronuclear envelope leads to rapid accumulation of cGAS, exposing self-DNA to the cytosol. cGAS binds to and is activated by chromatin, and its localization to micronuclei is cell-cycle dependent. By combining live-cell laser microdissection with single-cell transcriptomics, the study shows that interferon-stimulated gene expression is induced in micronucleated cells. Micronuclei represent an important source of immunostimulatory DNA. Since micronuclei formed from lagging chromosomes also activate this pathway, cGAS recognition of micronuclei may act as a cell-intrinsic immune surveillance mechanism detecting a range of neoplasia-inducing processes. DNA is a key pathogen-associated molecular pattern sensed by innate immune receptors in the cytosol and endosomal compartments. Strict compartmentalization of cellular DNA in the nucleus and mitochondria is necessary to avoid sensing of self-DNA. cGAS is a major cytosolic nucleic acid sensor with dsDNA as its canonical ligand. cGAS activation generates the cyclic dinucleotide cyclic GMP-AMP (cGAMP), which activates a Type I Interferon response via the adaptor Stimulator of Interferon Genes (STING). Aberrant recognition of immunostimulatory cytosolic DNA has been implicated in neoplasia and systemic autoinflammatory disease, with cGAS/STING-dependent inflammation associated with mutations in multiple nucleases. One such nuclease, RNase H2, maintains mammalian genome integrity through its role in ribonucleotide excision repair. This suggests that endogenous DNA damage may generate the nucleic acid ligands sensed by cGAS. Micronuclei occur at high frequency in Rnaseh2b-/- mouse embryonic fibroblasts (MEFs) compared with Rnaseh2b++ MEFs. This led to the consideration of micronuclei as a potential source of immunostimulatory DNA. Micronuclei, surrounded by their own nuclear envelope, arise during mitosis from lagging chromosomal DNA and chromatin bridges as a consequence of unresolved genome instability. Increased micronuclei formation was also observed in RcGAS surveillance of micronuclei links genome instability to innate immunity DNA is strictly compartmentalized within the nucleus to prevent autoimmunity. Despite this, cGAS, a cytosolic sensor of dsDNA, is activated in autoinflammatory disorders and by DNA damage. The mechanism by which cellular DNA gains access to the cytoplasm remains unclear. This study shows that cGAS localizes to micronuclei arising from genome instability in a model of monogenic autoinflammation, after exogenous DNA damage and spontaneously in human cancer cells. These micronuclei, which occur after mis-segregation of DNA during cell division, consist of chromatin surrounded by their own nuclear membrane. Breakdown of the micronuclear envelope leads to rapid accumulation of cGAS, exposing self-DNA to the cytosol. cGAS binds to and is activated by chromatin, and its localization to micronuclei is cell-cycle dependent. By combining live-cell laser microdissection with single-cell transcriptomics, the study shows that interferon-stimulated gene expression is induced in micronucleated cells. Micronuclei represent an important source of immunostimulatory DNA. Since micronuclei formed from lagging chromosomes also activate this pathway, cGAS recognition of micronuclei may act as a cell-intrinsic immune surveillance mechanism detecting a range of neoplasia-inducing processes. DNA is a key pathogen-associated molecular pattern sensed by innate immune receptors in the cytosol and endosomal compartments. Strict compartmentalization of cellular DNA in the nucleus and mitochondria is necessary to avoid sensing of self-DNA. cGAS is a major cytosolic nucleic acid sensor with dsDNA as its canonical ligand. cGAS activation generates the cyclic dinucleotide cyclic GMP-AMP (cGAMP), which activates a Type I Interferon response via the adaptor Stimulator of Interferon Genes (STING). Aberrant recognition of immunostimulatory cytosolic DNA has been implicated in neoplasia and systemic autoinflammatory disease, with cGAS/STING-dependent inflammation associated with mutations in multiple nucleases. One such nuclease, RNase H2, maintains mammalian genome integrity through its role in ribonucleotide excision repair. This suggests that endogenous DNA damage may generate the nucleic acid ligands sensed by cGAS. Micronuclei occur at high frequency in Rnaseh2b-/- mouse embryonic fibroblasts (MEFs) compared with Rnaseh2b++ MEFs. This led to the consideration of micronuclei as a potential source of immunostimulatory DNA. Micronuclei, surrounded by their own nuclear envelope, arise during mitosis from lagging chromosomal DNA and chromatin bridges as a consequence of unresolved genome instability. Increased micronuclei formation was also observed in R