<p>Gears are machine elements that transmit power and motion. They are dynamic components; thus, failure of a gearbox can cause failure of the entire system in which the gearbox is working. Therefore, understanding gearbox failure is an important task for the researchers in this field. There are numerous studies in literature to discover the failure reasons of gears. Knowing all types of failure has a vital importance to uncover the reasons of failure. Moreover, destructive/nondestructive techniques to be used are also vital to bring out the causes of failure in objective means. In this context, gear failure types and mechanisms leading to failure are presented comprehensively. Destructive/nondestructive techniques are explained systematically. Then, in the light of knowledge in failure types and evaluation methodologies, gear failure analysis examples from different industries are examined. The purpose of this review is to organize gear failure analysis in a systematic way to help the other researchers understand the subject in a practical way. It is believed that the current study differs positively from other existing studies since it addresses the subject in all its aspects and in a holistic manner.</p>