Detailed vibration measurements were taken on the Clifton Suspension Bridge in Bristol, England to define its basic dynamic characteristics in normal conditions and then to assess its dynamic response to crowd loading; 27 vibration modes were identified with natural frequencies below 3 Hz. When subject to crowd loading, quite large lateral vibrations occurred in two modes, with sudden onset. This phenomenon, often termed ‘synchronous lateral excitation’ or ‘pedestrian lock-in', is similar to the behaviour observed on the London Millennium Bridge and a number of other bridges. Data analysis showed the behaviour to be consistent with the pedestrian negative damping model proposed by Arup in developing a solution for the Millennium Bridge. This model does not, however, explain the underlying mechanism causing the excitation, and a number of observations of the behaviour of the Clifton Suspension Bridge suggest that significant synchronisation of pedestrians did not actually occur. Although synchronisation may occur for large-amplitude vibrations on some bridges, the observations challenge the commonly held view that this mechanism is responsible for the initial rapid onset of lateral vibrations due to crowd loading.