The dynamic behaviour of the Second Severn Crossing cable-stayed bridge was monitored throughout the construction period. This paper presents the results of a modal survey, conducted in ambient conditions, of a completed cantilever before closure of the main span. A finite element model of the structure is described, and the results of the measured and modelled modal behaviour are compared. Good agreement was found between the site-measured modes and those predicted by the finite element model for vibrations in the vertical/longitudinal plane, however there were greater discrepancies for modes involving torsion. Work is continuing to improve the accuracy of the model and to investigate more detailed aspects of the dynamic behaviour of such bridges.