This book deals with the synthesis and characterization of NiO deposited by Chemical Bath Deposition method using solution of Ni(NO3)2.6H2O. X-ray diffraction investigation showed cubic crystal structure of NiO with preferred orientation along (200). The scanning electron microscopy images exhibited a porous structure morphology composed of nanosheets. The network of linked nanosheets and voids offers a large number of pathways and more reactive sites for gas diffusion. TEM image reveals an average size of 5 nm for NiO. AFM reveals roughness of 107.9 nm for NiO deposited at 7 h. BET measurement gives the specific surface area of 110.42 m2g-1. The XPS study showed that the presence of mixed Ni2+ and Ni3+ chemical states of Ni at about 853.70 and 855.44 eV, respectively on the NiO surface due to non-stoichiometry of the film. The direct band gap energy of around 3.6-3.7 eV was obtained by UV-Vis measurement for all NiO films. 45.6% response is obtained to 100 ppm NO2 at 200 C with T_response = 13 s and T_recovery = 146 s. The highest response of 45.6% to 100 ppm NO2 showed that NiO is a good candidate for environmental monitoring at moderate working temperature of 200 C.