This paper presents a review of different modes of gears failure. Gear is the most important mechanical element for transmitting uniform and smooth power. Gears are used in almost every power producing and transmitting systems, i-e automobile, aircraft, railway, computers etc. During transmission of power the critical location is gear tooth, because even if one gear tooth fails, then that gear is of no use. The gear tooth profile is very important for transferring power uniformly and smoothly. Even if gear tooth do not fail (tooth breakage) completely, but tooth surface profile change then its desire job cease. Failure of a gear during operation results into discontinuity in the operating system, inefficient, uneconomical system and even can results into human damage. In this regard a number of tooth complete failure (tooth breakage) and tooth surface profile failure has been reported theoretically and experimentally from literature. The frequently mode of gear failure is fatigue failure, i-e both fatigue bending stress and fatigue contact stress. The gear tooth breakage is because of fatigue bending stress and sudden overload, while the tooth surface failure is mainly because of fatigue contact stress. There are also some other surface failure phenomenon like scoring, surface wear and plastic flow. Misalignment of power transmitting shafts, shaft gear assembly, and loss of lubrication are contributing factors towards gear failure modes. One by one each gear failure mode is discussed briefly with available theoretical and experimental results. At the end of paper all gear failure modes are summarized.