This chapter presents some examples of classic gas sensors. A sort of gas-sensitive material is cited followed by the principles of optical gas sensors. The chapter introduces some successful plastic optical fiber (POF) gas sensors. Gas sensors are a subcategory of chemical detectors, which are designated to sense and/or quantify the concentration of one or more gaseous species. Monitoring of some gas species, which influence the air quality, is crucial in both work ambient and indoor well-being care. Gas detectors are in action right after the discovery of hazardous effects of some species such as carbon monoxide and hydrogen sulfide. Plastic optical fibers have a series of advantages such as ease of handling, low precision connectors, and mechanical resistance. A large number of POF sensors operate based on light intensity variation through transmission, reflection, spectroscopic, and evanescent field transduction mechanisms.