  This presentation discusses some recently active topics in the theoretical
interpretation of high energy heavy ion collisions at the LHC and at RHIC. We
argue that the standard paradigm for understanding the spacetime evolution of
the bulk of the matter produced in the collision is provided by viscous
relativistic hydrodynamics, which can be used to systematically extract
properties of the QCD medium from experimental results. The initial conditions
of this hydrodynamical evolution are increasingly well understood in terms of
gluon saturation, and can be quantified using Classical Yang-Mills fields and
QCD effective kinetic theory. Hard and electromagnetic probes of the plasma
provide additional constraints. A particularly fascinating subject are high
multiplicity proton-proton and proton-nucleus collisions, where some of the
characteristics previously attributed to only nucleus-nucleus collisions have
been observed.
