The book demonstrates the synthesis process of metal oxide semiconductor and Graphene/rGO for gas sensor applications with a motive to development of efficient room temperature hydrogen gas sensor using n-ZnO/p-rGO heterojunction nanohybrid structure in support of the growing demand for the low cost, simple fabrication, long-lasting, stable, exceptionally sensitive, and selective hydrogen gas monitoring system particularly to ensure industrial safety.The first part of the book discusses the gas sensitive thin films of zinc oxide (ZnO) and studies the sensitivity behavior of the developed films towards hydrogen detection. A low cost chemical deposition technique has been used to deposit the ZnO NPs thin film. This book studies have been done on the sensor response under different gas concentrations. A low-cost simple electrochemical exfoliation technique has been used to synthesis Graphene /rGO. Raman spectra determine the crystallinity, as well as the number of graphene layers, present in the sample.