There is experimental evidence that the quark-gluon plasma produced in ultra-relativistic heavy ion collisions is well described by viscous hydrodynamics, with a low value of the viscosity (relative to the entropy density). This observation, added to the recent discovery that the same description works well also for high energy proton-nucleus or high multiplicity proton-proton collisions, is raising a number of interesting theoretical questions: How does the system of gluons freed in the early stage of a collision evolve towards local thermal equilibrium? How does hydrodynamical behavior emerge in systems whose natural description is in terms of quantum field theory? Can we observe hydrodynamical behavior in the absence of local equilibrium? Such questions, among others, will be discussed during the talk.