The need, in many cases, to know the tension of a cable in a structure led to seeking a simple and rapid method of measurement. This method is based on the theory of vibrating strings; it enables the tension to be obtained from the measurement of the frequency of vibration of the cable and a knowledge of its length and mass per unit length. The authors justify the application of the 'vibrating string' model to a cable, and specify the limits of its field of utilization. A simple rule has been established to determine whatever a given case lies within this field. The tension can be measured to an accuracy of better than 5%. Examples are given of the application to bridges incorporating prestressed cables. The authors also indicate the principal precautions to be taken for the correct use of this method.