Abstarct: Considering the growing serious environmental issues of air pollution, developing reliable gas sensors for accurate trace detection of toxic substances in the air has become one of the world's most serious concerns. In order to tackle this challenge, there has been thorough studies and investigations performed to introduce and improve semiconductor gas sensors especially SnO2 gas sensors. To achieve this goal it is vital to study the gas-sensing process and quantum simulations of sensors. The results predicted by theoretical simulations can support the experimental measurements and provides a better understanding of the sensing mechanism. Therefore, the main goal of this dissertation is to theoretically predict and experimentally synthesize to investigate electronic, optical, and sensing properties of tin dioxide nanostructures. In this study firstly, the SnO2 (110) surface has been simulated and its structural, electronic, optical, and sensing properties have been investigated using first-principles calculations baxsed on density functional theory (DFT) within generalized gradient approximation (GGA). SIESTA and Quantum ESPRESSO packages. Then the adsorption of CO2, Ch3OH, O2 molecules on the SnO2 laxyer and its different properties including electronic structure, band structure, lattice parameters, adsorption energy, and charge transferred in the adsorption process have been studied. Besides, in the experimental part of the project SnO2 nanostructured thin film sensors fabricated by solvothermal, sputtering, and thermal evaporation deposition techniques. The synthesized nanostructures were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscope (FESEM), Fourier-transform infrared spectroscopy (FTIR), and UV-vis. At the next step, the sensing properties of the samples was measured towards CO2, Ch3OH, and O2. In the end, there will be an evaluation of the credibility of both experimental and theoretical results, by comparing and interpreting them.