In this chapter, first the fundamental law of gearing is introduced. This law is an application of the angular-velocity-ratio theorem (discussed in Chap. 4 ) on gears. The relation between the fundamental law of gearing and the angular-velocity-ratio theorem provides a bridge between the instantaneous centers topic and the gears discussion. Then, gear terminology, related equations, and gear standards are introduced. The use of involute of a circle for maintaining constant speed ratio in two gears in mesh is discussed. Next, design methods for preventing interference and undercutting in meshed gears are presented. In this way, we make sure that in a gearset, motion is transmitted at constant speed ratio. Chapter 6 is concluded by quantifying contact ratio in a gear mesh and evaluating the effect of a change in the center distance of gears, on factors like pressure angle and backlash.