Exceptional physical properties of graphene and possibilities of its electronic applications have evoked a great deal of research interest for the last few years. From a theoretical point of view, electrons in graphene behave like massless Dirac fermion and equivalent to a massless neutrino in two dimensions. Therefore, mobility of electrons in graphene is the highest among all solids. Graphene exhibits ballistic charge transport properties. Graphene is actually a semimetal. However, the lack of an energy gap in its electronic spectra, for example, prevents the use of graphene in making transistors, logic circuit devices, and sensors. Several methods have been developed to break the lattice symmetry in graphene and open an energy gap. The opening of the bandgap in graphene is a unique phenomenon. Several versions of gap opening are discussed in the chapter after a short introduction on the synthesis procedures of graphene and graphene oxide. The best surface-to-volume ratio is in graphene-like two-dimensional (2D) materials, every atom of which may be considered as a surface atom; therefore, the use of graphene has opened up the opportunity for the detection of gases and vapors with excellent sensitivity, fast response time, and high stability. Different types of sensors are considered. Besides resistive sensors, eld-effect transistor, and acoustic wave and functionalized gas/vapor sensors have been reported.