Nonlinear energy sinks (NESs) are a type of passive mass damper utilizing essential nonlinear stiffness elements, generally with a cubic relationship between displacement and their restoring force. The essentially nonlinear restoring force provided by these elements allows the NES to interact with any mode of the primary structure to which the NES is attached and participate in targeted energy transfer (TET), the broadband transfer of energy from the primary structure to the NES where it can be rapidly dissipated. Additionally, this nonlinear restoring force allows the linear modes of the primary structure to become coupled and energy to be transferred from the lower modes of vibration to the higher modes where it is dissipated at a faster rate. Research is ongoing within the group studying the use of NESs to mitigate vibration in civil structures caused by both extreme loads and service level loads.