Significant aspects of gear life performance related to stress/strength relationships and the subtleties of microstructure are discussed. Competitive failure modes are described in relation to gradient stresses and gradient strengths. With a focus on carburized steel, criteria related to material are defined, e.g., case and base hardenabilities, along with their importance in developing microstructure and hardness gradients. Some discussion is also presented on wear and scoring phenomenon. Comments are included on recent information on carbonitriding applied to gears and austempered ductile iron as a gear material. It is a review drawing heavily from existing literature.