<p>ZnO has a long history as a material for gas sensing, varistors, piezoelectric transducers, as a light-transmitting electrode in optoelectronic devices, electro-optic modulators and as a sunscreen. As a gas sensor, ZnO and doped ZnO materials have been invensively used. This chapter looks at the importance of chemical solution-based technologies and the SILAR method on the gas-sensing properties in details. We believe that a combined understanding of theoretical and experimental aspects will open new fields for metal oxide gas sensor and beyond.<br></p>