Hydrogen and internal stress failures are relatively rare; however@ when they occur they are often very costly and sometimes quite catastrophic. While hydrogen and internal stress issues are generally recognized as significant in the design and manufacture of larger gears@ they are also important for smaller gears as well. This paper presents@ via illustrated actual case studies@ the mechanisms by which these failure occur@ the manner in which they progress@ and methods for testing finished gears for the possibility of internal problems. In addition@ precautionary steps that can be taken during design@ manufacture@ heat treatment and quality control to minimize the possibility of these problems occurring in a finished part along with similar steps required to prevent any flawed gears from entering service are also presented and discussed.