Wind energy is presently the fastest growing renewable energy source in the world. However, the industry still experiences premature turbine component failures, which lead to increased operation and maintenance (O&M) costs and subsequently, increased cost of energy (COE). To make wind power more competitive, it is necessary to reduce turbine downtime and increase reliability. Condition monitoring may help by reducing the chances of catastrophic failures, enabling cost-effective operation and maintenance practices, and providing inputs to improve turbine operation, control strategy, and component design. As compared with other applications, the wind industry started recognizing the benefits and importance of condition monitoring relatively late. However, interest has increased so much that a workshop was organized by the National Renewable Energy Laboratory (NREL) in 2009 as a response. This paper provides an overview of wind turbine drivetrain condition monitoring based on workshop presentations and additional references. Since the gearbox has been shown to have the longest downtime and is the most costly subsystem to maintain throughout a turbine’s 20 years of design life, it has been chosen as the main targeted subsystem of this study. Wind turbine drivetrain condition monitoring practices, challenges, and future research opportunities will be addressed in detail.