  I review recent developments in the phenomenological study of the quark-gluon
plasma (QGP) transport properties based on a personal selection of results that
were presented at Quark Matter 2019. The constraints on the temperature
dependence of QGP shear and bulk viscosity are summarized. I discuss new theory
advancements towards more realistic 3D dynamical simulations of heavy-ion
collisions at finite baryon density. The challenges and opportunities of
applying hydrodynamics to small collision systems are highlighted.
