This chapter reviews the gas sensing properties of the carbon nanotubes (CNTs) at the current state of art. The physical properties of the CNTs are an outstanding mix for advanced nanosensors at high sensitivity and very low sub-ppm level of gas detection at room temperature. Fabrication techniques of the CNTs are surveyed as well. Pristine, modified, purified, and functionalized CNTs are considered as chemically interfaced materials for gas adsorption by using various transducers as innovative platforms. Here we review the key developments of the CNTs gas sensors by means of a comparative analysis of the chemical sensing performance and outline promising applications, challenges, and future perspectives. © 2013 Pan Stanford Publishing Pte. Ltd. All rights reserved.