The theory of self-loosening of preloaded bolted connections when subjected to vibration is discussed. The significance of self-loosening as a cause of failure is explained, and design guidance to avoid self-loosening is given. The test methods are described and discussed in connection with a newly designed testing machine that yields quantitative data for evaluating locking properties. These methods can be applied to all types of locking elements. Finally, a simplified method for broad scale testing and inspection is proposed.