Plasmonic gas sensors are optical sensors that use localized surface plasmons or extended surface plasmons as transducing platform. Surface plasmons are very sensitive to dielectric variations of the environment or to electron exchange, and these effects have been exploited for the realization of sensitive gas sensors. The localized surface plasmon resonance of Au nanoparticles (NPs) is sensitive to both dielectric constant changes in the supporting matrix and to changes in electron density of the metal NPs and both these effects have been exploited for gas sensing applications. Au NPs embedded in different matrices (TiO2, NiO, ZnO, WO3) showed highly sensitive, and reversible changes in optical absorption when exposed to H2, NOx, H2S and CO species at 250°C. If Pt NPs are added, the thin films showed room-temperature responses to H2. Moreover, dark field microscopy has been used for direct observation of the kinetics of H2 interaction with single gold nanorods.