  The latest measurements of collective behaviour in a variety of collision
systems with the ATLAS detector at the LHC, including pp collisions at 13 TeV,
Xe+Xe collisions at 5.44 TeV, and Pb+Pb collisions at 5.02 TeV, are presented.
They include vn-[pT] correlations, which carry important information about the
initial-state geometry of the quark-gluon plasma and can shed light on any
quadrupole deformation in the Xe nucleus, and measurements of flow
decorrelations differential in rapidity, which probe the longitudinal structure
of the colliding system. These measurements furthermore provide stringent tests
of the theoretical understanding of the initial state in heavy ion collisions.
