Contact stress is generally the deciding factor for the determination of requisite dimensions of gears. Research on gear action has confirmed the fact that contact pressure influences the formation of pits on the tooth surface and the other defects such as scoring, interference and undercut. Hertzian equations are generally used for investing surface pitting and scoring by obtaining the magnitude of the contact stresses. A Spur gear is an important machine element which is used extensively for transmitting motion and power between shafts. The complexity of the Spur gear tooth is that its thickness and height vary from heel to toe which results in a non-uniform distribution of load along the face width of the tooth. Therefore, a three-dimensional analysis is needed for an accurate study of the Spur gear tooth. The major objective of this book is how to design the gear with the consideration of stress.