  In this contribution I discuss the nuclear symmetry energy in the regime of
hadronic degrees of freedom. The density dependence of the symmetry energy is
important from very low densities in supernova explosions, to the structure of
neutron-rich nuclei around saturation density, and to several times saturation
density in neutron stars. Heavy ion collisions are the only means to study this
density dependence in the laboratory. Numerical simulations of transport
theories are used to extract the equation-of-state, and thus also the symmetry
energy. I discuss some examples, which relate particularly to the high density
symmetry energy, which is of particular interest today. I review the status and
point out some open problems in the determination of the symmetry energy in
heavy ion collisions.
