There are enormous functional materials which have been explored for gas sensing applications due to the fact that when a particular gas interacts with their surfaces, some alteration in their properties (optical, electrical, etc.) is observed. These functional materials are also termed as sensing films, to be utilized as one of the essential components in a gas sensor. Other components include electrodes connected with sensing film and data acquisition system coupled with it. During the past several decades, various metals, semiconductors, ceramics, and hybrid materials have been extensively explored for gas sensing applications. Selection of functional film is dependent on the gas to be detected. A gas sensor should have some characteristics, viz., higher sensitivity, selectivity for target gas, least response and recovery time, higher reproducibility, and stability. Therefore, selection of appropriate sensing film is highly required for a well-efficient gas sensor development. This article reviews the various kinds of sensing films, their fundamental aspects along with the sensing mechanisms. Morphological changes in the materials and doping of other functional materials also affect the performance of a gas sensor. Hence, issues related to the efficient gas sensing are also covered in this article. © 2019, Springer Nature Singapore Pte Ltd.