  We investigate the density dependence of the symmetry energy in a
relativistic description by decomposing the iso-vector mean field into
contributions with different Lorentz properties. We find important effects of
the iso-vector, scalar $\delta$ channel on the density behavior of the symmetry
energy. Finite nuclei studies show only moderate effects originating from the
virtual $\delta$ meson. In heavy ion collisions from Fermi to relativistic
energies up to $1-2 AGeV$ one finds important contributions on the dynamics
arising from the different treatment of the microscopic Lorentz structure of
the symmetry energy. We discuss a variety of possible signals which could set
constraints on the still unknown density dependence of the symmetry energy,
when experimental data will be available. Examples of such observables are
isospin collective flow, threshold production of pions and kaons, isospin
equilibration and stopping in asymmetric systems like $Au+Au$, $Sn+Sn$ and
$Ru(Zr)+Zr(Ru)$.
