Vector boson scattering (VBS) is a key process to study at the high-energy frontier. In particular longitudinal polarisation states of the boson have an intrinsic connection to the electroweak symmetry breaking and the Brout-Englert-Higgs mechanism. The Large Hadron Collider provides an unprecedented high-statistics dataset at the highest energy scale, enabling a comprehensive study of VBS, and search for deviations from Standard Model predictions. In this seminar, the first evidence of longitudinal boson polarisation in VBS through the study of electroweak production of same-sign W boson pairs obtained by ATLAS will be presented.  The analysis techniques and innovations important for achieving this result will be discussed. In addition, the seminar will recap several ATLAS measurements of (longitudinal) boson polarization with inclusive diboson processes, which shared the path to establish the methodology of boson polarization measurement as well as to pursue next-level precision tests of the Standard Model.