The thermal treatment of metals can include such processes as through hardening, surface hardening, annealing, stress relieving and sintering to name a few. In some instances the performance of the component in service can be adversely affected by errors committed during heat treatment. Optical microscopy of metallographic cross sections is often used alone or in conjunction with other tests to determine the cause of the poor performance. The following gives a few examples of actual case histories of failed components that have been examined by the author.