During the past few decades gas sensors have become more and more important in several domestic, urban, and industrial environments. The discovery of novel nano-structures provided ideal building blocks to fabricate a new generation of sensors that show significantly improved sensitivity and greatly reduced volume. The tiny size and dimensionality of nano-structures produce new properties (larger surface-to-volume ratio, higher reactivity, lower working temperature...) leading to better intrinsic performance. This book shows how metal oxide nano-structures and organic oligomers can be used to fabricate effective gas sensors. The different parameters used to evaluate the performance of a sensing device are elucidated. The growth mechanism of metal oxide nano-wires is described, as that of quasi-crystalline organic thin films nucleation and growth. The depletion layer modulation model is explained and proven with experimental data from single nano wire devices. The last chapter shows a possible approach to the novel concept of hybrid sensors.