Castle Bridge is a contemporary and slender S shaped footbridge linking Finzels Reach with Bristol city centre. Full Scale Dynamics were contacted by CTS Bridges at the design stage to perform a vibration serviceability assessment to check the performance of this bridge when subject to groups of walkers, joggers and crowds of people. Due to the extremely slender design of the footbridge, it was found to be particularly susceptible to large vibrations under jogging conditions and crowds, so a Tuned Mass Damper was proposed as a solution that would reduce the accelerations to an acceptable level in all scenarios. Full Scale Dynamics worked with CTS Bridges to design and fabricate a Tuned Mass Damper that could be installed under the bridge deck. Upon installation, the performance of the bridge with and without the 600kg moving mass Tuned Mass Damper was tested by Full Scale Dynamics in subsequent dynamic testing on-site. This testing showed the significant reductions in vibrations achieved, bringing the bridge in line with required Eurocode standards.