The evolution of nanotechnology has recently introduced the synthesis of crystalline nanostructures, usually referred to as nanowires, nanorods or nanobelts, which share the feature of very small lateral dimension and extremely high aspect ratio. Metal oxide nanowires and their great functional potential are attracting a great interest as they exhibit physical properties which are significantly different from the polycrystalline counterpart owing to the confinement effects arising from the nanosized lateral dimensions. This chapter summarizes the established descriptive model of gas sensing with polycrystalline nanostructures of transparent conducting oxides (TCOs) such as tin oxide and indium oxide and highlights the impact of the introduction of single crystalline nanowires on sensor technology.The chapter also deals with the methodology of fabrication and the technological issues concerning control of dimension, homogeneity and uniformity of the nanowires. The basic mechanism driving the nucleation and growth is discussed starting from the different strategies presently adopted for synthesis. In general, TCO nanowires are single crystalline structures with well-defined and uniform chemical composition, surface terminations, free from dislocation and other extended defects.