Molybdenum oxide (MoO3) is a material finds potential applications due to its abundance and attractive properties such as catalyst, gas sensing and electrochromism. MoO3 thin films exist in three phases namely: orthorhombic - phase, monoclinic beta- phase and hexagonal phase. The crystal structure of beta- MoO3 in which MoO6+ octahedral form a three dimensional network by sharing the corners with each other and well suited as a catalyst for number of chemical reactions. Recently, nanocrystalline oxide films attracted much for their promising applications in building of nanodevices and nanosensors. MoO3 films in nanocrystalline form also find applications in sensors and lubricants.This book restrains the optimized conditions of pure MoO3 thin films on glass and silicon substrates using RF-Magnetron sputtering and investigated the structural, morphological,optical, electrochromic properties and sensor applications.