Nickel oxide (NiO) is a transition metal oxide semiconductor, and is a Mott Hubbard insulator that crystallizes in a rock salt structure and it is considered to be a model semiconductor with p-type conductivity due to holes generated by Ni vacancies in the lattice. In recent years, NiO has received a great interest due to its unique properties and is an important material for use in device applications such as gas sensors, dye sensitized photocathodes and electrodes in alkaline batteries. The promising approach to prepare device quality pure and copper doped NiO nanoparticles for gas sensor applications was discussed clearly in this book.