This chapter reviews the air quality sensors distributed in urban areas for wireless node network applications at the current state of art. The functional materials such as metal oxides, carbon nanomaterials, and hybrid composites are enabling technologies for chemical sensing. Low-cost sensors integrated in low-power sensor systems are operated in wireless sensor networks for urban air quality monitoring of toxic gases and particulate matter at outdoor level with high spatial and temporal resolution. Examples of stationary sensor networks and mobile sensing on ground vehicles and unmanned aerial vehicles (UAV) are presented in terms of smart and sustainable cities and Internet-of-Things (IoT) applications as supplement of the expensive air quality monitoring stations. The air quality sensor performance of the existing worldwide-operated wireless sensor networks is described with emphasis to the data quality objectives. The article concludes on challenging applications and promising perspectives of the air quality sensors in the rapidly emerging fields. © 2020 Elsevier Inc. All rights reserved.