Geological CO 2 storage is transitioning from demonstration to industrial-scale deployment, and faults within and around storage complexes exert a first-order control on its safety and performance. Faults can compartmentalise reservoirs through across-fault sealing, provide pathways for vertical migration of CO 2 and brine across caprocks, and host induced seismicity when reactivated by pore-pressure, thermal and geochemical changes during and after injection. These behaviours are not independent in the sense that the same fault can seal hydraulically while remaining critically stressed, or slip without becoming transmissive, and its state can evolve over engineering to geological timescales. We synthesise structural geology, fault-seal analysis, geomechanics, geochemistry, modelling and the operational record from projects including Sleipner, In Salah, Decatur, Quest, Snøhvit, Aquistore and Gorgon. No measurable fault-related leakage from a storage complex has been documented at any saline-aquifer or depleted-gas-field project. Microseismicity on unmapped sub-seismic and basement structures is recurrent, and in 2026 two events above M L 4.0 near Quest became the first felt events at a storage project, provisionally attributed to injection and without effect on containment. Because the pressure and stress perturbation extends far beyond the CO 2 plume, the faults that host seismicity commonly lie beneath or outside the storage complex and loading there can peak after injection stops. We review across-fault sealing proxies and their CO 2 -specific uncertainty, fault-fracture network controls on leakage, induced-seismicity mechanisms, natural analogues, regulatory frameworks and post-injection behaviour, and identify five priority data classes and the coupled workflows needed to convert them into defensible risk envelopes.