Abstract: A method of monitoring the tension in a flexible drive belt (1) which passes around a plurality of pulleys (2, 3) includes the steps of causing a portion of the belt (1) when installed around the pulleys to vibrate at a natural frequency and detecting that frequency. In one arrangement the belt (1) is set into a vibrating motion by an impulse and the frequency of the transient vibrations measured. In another arrangement the belt (1) vibrated by a drive mechanism which receives a drive signal derived from the belt movement. Specific methods and apparatus for monitoring the tension are also applied to measuring the tension in elongate flexible members other than drive belts.