This chapter describes the macro and microgeometry optimization of asymmetric tooth gears. Gear pair size reduction is typically limited by the tooth flank surface endurance defined by pitting and scuffing resistance. Asymmetric tooth profiles make it possible to increase the operating pressure angle beyond the conventional symmetric gear limits. The contact ratio of spur gears is a critical parameter that affects the gear drive performance. It influences the gear drive load capacity, efficiency, and vibration. The gear tooth root fillet profile is typically a trochoidal curve determined by generating tool tooth tip trajectory. Direct Gear Design optimizes a tooth root fillet after the involute flank parameters are already completely defined. Thin rim gears naturally have higher bending root stress compared to the solid body gears. Gear mesh power losses depend on the gear tooth geometry and friction coefficient. A relative angular position between the planet gear teeth and supporting bearing roller is constantly changing during the epicyclic stage operation.