Application of a bearing steel requires a knowledge of how the steel will change size at the temperatures of bearing operation, to determine if the changes in geometry will effect bearing performance. The concern is greatest for through hardened steels whose entire volume is subject to expansion from transformation of retained austenite and contraction from tempering of martensite. This paper presents size change data for 52100 and 52100 Mod. II steels for aging temperatures from room temperature to 220°C (425°F), and for M50 and stainless bearing steels aged up to 425°C (800°F). Historic papers establishing the fundamentals of stability in low alloy steels are reviewed. Behavior of high alloy and stainless bearing steels is explained.