During the last century, many semiconductor gas sensors have been presented based on various materials and technologies to detect gas components. Many of them are made to support human's life in different ways. In the last years, the number of sensors which can detect various gas species has increased dramatically especially with respect to global issues of environment and earth atmosphere, the necessity of those gas sensors which can detect air pollutants such as NO x gases in environment is felt. They are essentially required in order to control the systems of combustion exhausted from industry, stationary facilities and automobiles. So it is necessary for the scientists to design and fabricate the gas sensors which are of low cost, selective, sensitive, and accurate especially in low level concentrations of target gases, easy to process and of high recovery and response time, good electrical properties, and above all tunable structure at the nano scale. In this sense, this paper has presented various classifications of gas sensors and various semiconductor oxides for NO x detecting and their technology to convey a comprehensive idea about the semiconductor metals as well as metal oxides for NO x detecting along with their characteristics, structures etc. in order to clarify the future road map towards on the semiconductor materials for NO x detecting. Keywords: sensor, NO x gases, semiconductor, metal oxides INTRODUCTIONIn the last few decades, many studies have been focused on the semiconducting metal oxides which are known as effective gas sensing materials especially for NO x detecting due to their electrical conductivities which are open to change greatly with the changes of atmosphere (K. J. Choi & Jang, 2010;Comini et al., 2009;Fine, Cavanagh, Afonja, & Binions, 2010;Franke, Koplin, & Simon, 2006; Kanan, El-Kadri, Abu-Yousef, & Kanan, 2009; G Korotcenkov & Cho, 2011;J.-H. Lee, 2009;Tricoli, Righettoni, & Teleki, 2010;Wetchakun et al., 2011). The sensing characteristics of such sensors can be optimized by controlling their morphology, surface to volume ratio, and electrical properties. The developments in nanotechnology, fabrication methods, and synthesis provide a suitable atmosphere to construct those sensors with their mentioned sensing characteristics easier.