Currently there are two main reasons for seeking new methods and technologies that aim to develop new and more perfect sensors detecting various chemical compounds. The first reason is man's striving for an ever better understanding of the surrounding world and the universe. Second, sensors are used to ensure safety, e.g. in the vicinity of factories, in an important objects like airports, in environmental protection, health care, etc. These applications have a significant impact on the performance of sensors. This chapter addresses the issue of some nitrogen oxides (NOx) sensor designs using some of the most sensitive methods such as cavity enhanced absorption spectroscopy (CEAS) and cavity ring down spectroscopy (CRDS). Nitrogen oxides are compounds of nitrogen and oxygen. For example, among them very important are nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O). According to the HITRAN database, in standard atmosphere1 their concentration is as follows: NO – about 0.3 ppbv2, NO2 – about 0,023 ppbv, N2O – about 320 ppbv. However, in real ambient air their concentrations are strongly related to meteorological conditions and emission sources (anthropogenic and natural). They are compounds that play a significant role in many different fields. They are important greenhouse gases, and their reactions with H2O (water) lead to acid rains. For example, nitrous oxide is used as an anaesthetic, especially in dentistry and minor surgery. It produces mild hysteria and laughter. Thus it is also known as ‘laughing gas’. Atmospheric photochemistry induces a complicated conversion mechanism between nitrogen oxides [Godish, 2004]. Moreover, NO, NO2 and N2O are also characteristic decomposition compounds which are the main products of specific explosives materials. Many of them contain NO2 groups, which can be detected using spectroscopic detection methods [Moore, 2007]. There are many methods for NOx detection. For example, in the case of gas chromatography and mass spectrometry, a detection limit of a few dozen ppb is reported