This chapter discusses nanocomposites based on carbon nanotubes (CNTs) and metal oxides (MOX) for gas sensing applications. The chapter first reviews the historical background of the MOX and the hybridized MOX/CNT composite gas sensors. It then describes several popular methods for preparation of MOX/CNT sensing films such as spin-coating, drop-coating, screen-printing, dip-coating, and electron beam evaporation techniques. The ways to characterize MOX/CNT materials with their basic theories including raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been presented. Finally, the sensing mechanism of a MOX/CNT gas sensor based on p–n heterojunctions has been highlighted.