Tin dioxideTin dioxide (SnO2), an n-type wide bandgap metal oxide, has emerged as a cornerstone material for chemical sensingSensing owing to its robust redox surface chemistry, thermalThermal stability, CMOS compatibility, and the rich palette of nanostructuredNanostructures morphologies now accessible via solution and vapor-phase synthesis. This review consolidates advances in SnO2 nanostructuresSnO nanostructures forNanostructures electrochemical and photochemical (light-assisted/opticalOptical) sensingSensing. We outline fundamentals of surface charge modulation and depletion layer physics, defect/oxygen vacancy engineering, heterojunctionHeterojunction design, and noble-metal catalysisNoble-metal catalysis. After surveying synthetic routes (sol–gel, hydro/solvothermal, electrospinning, aerosol, CVD/VLS, templating), we connect morphology and defect landscapes to transduction pathways in amperometric/voltammetric and photoconductive/photocatalytic modalities. We critically examine strategies for sensitivity, selectivity, response–recovery kinetics, humidity mitigation, and room-temperature operation through UV/visible activationUV activation and plasmonic enhancement. ApplicationApplications vignettes span toxic gases (NO2, H2S, NH3), VOCs (ethanol, acetone), biomarkers (breath acetone, NH3), and liquid-phase analytes (H2O2, glucose, dopamine, heavy metals). We highlight progress toward flexible/wearable platforms, micro-hotplates, self-heated nanowiresNanowires, and data-driven selectivity using machine learning. Finally, we identify research gaps in long-term stability, humidity-robust selectivity, wafer-scale integration, and realistic field calibration, and sketch a roadmap for low-power, reliable SnO2 sensorSensors systems.