This chapter presents semiconductor chemoresistive gas sensors based on nanostructured metal oxides and sulfides, highlighting their operation through resistance changes induced by surface reactions with gases. It emphasizes the role of nanostructuring in enhancing sensitivity via increased surface-to-volume ratio. Key performance requirements, including sensitivity, selectivity, stability, and low power consumption, are outlined. The chapter also introduces sensors developed at the Sensor Laboratory of the University of Ferrara, describing their fabrication and functional properties.