In the last decades the interest in studying the dynamic properties of the structures in the field of civil engineering soared rapidly due to the fast development of large??scale civil structures. The analysis of the dynamic behaviour can reveal progressive damages as well as estimate the residual service life. Indeed, modal parameters and their evolution in time can be used as indicators and features of structural weaknesses or deficiencies induced by unforeseen events. The impossibility to excite large size structures with artificial devices and to record data in continuous fully controlled testing environments, leads to the use of output??only records. For this reason, different methods based on vibration data collected during working conditions have been proposed over the years such as the Operational Modal Analysis.