Bearing steels, which include high-carbon and low-carbon types, can be divided into service-based classes, such as normal service, high-temperature service, and service under corrosive conditions. This article discusses the importance of matching the hardenability and quenching of a bearing steel. It also discusses the typical microstructure of a high-carbon through-hardened bearing, and shows typical case and core microstructures in carburized bearing materials. Apart from a satisfactory microstructure, which is obtained through the proper combination of steel grade and heat treatment, the single most important factor in achieving high levels of rolling-contact fatigue life in bearings is the cleanliness, or freedom from harmful nonmetallic inclusions, of the steel. Alloy conservation and a more consistent heat-treating response are benefits of using specially designed bearing steels. The selection of a carburizing steel for a specific bearing section is based on the heat-treating practice of the producer, either direct quenching from carburizing or reheating for quenching, and on the characteristics of the quenching equipment.