Before studying how gear transmission works, we need to understand how motion is transmitted between a pair of links with curved surfaces in direct contact. These contact surfaces can be of any shape. Motion between them can be pure rolling, pure sliding or a combination of both. In this chapter we will learn about the kinematic requirements for a curve to be used to manufacture a gear and the commonly-used gear tooth shapes. We will study involute gears and their kinematics. Finally, we see a classification of toothed wheels and their fundamental concepts.