  In this contribution we briefly give an overview of the theoretical models
used to describe experimental data from heavy-ion collisions from
$\sqrt{s_{NN}} \approx $ 4 GeV to ultra-relativistic energies of $\sqrt{s_{NN}}
\approx $ 5 TeV. We highlight the successes and problems of statistical or
hadron-resonance gas models, address the results of macroscopic approaches like
hydrodynamics (in different hybrid combinations) as well as the results from
microscopic transport approaches in comparison to experimental data. Finally,
the transport coefficients like shear $\eta$ and bulk viscosity $\zeta$ -
entering the macroscopic models - are confronted with results from lattice QCD
in thermal equilibrium for vanishing chemical potential.
