This presentation discusses epicyclic gear trains and their applications. It describes the basic components and operation of simple and compound epicyclic gear trains, including sun and planet gears. Methods for calculating velocity ratios in epicyclic gears are presented. Examples of specific epicyclic gear applications discussed include Humpage's speed reduction gear and the differential gear used in automobiles. The advantages of epicyclic gears over conventional gears are their smaller size, higher durability, and ability to achieve high transmission ratios. Their disadvantages include higher cost and more complex design.