  The nuclear symmetry energy as a function of density is rather poorly
constrained theoretically and experimentally both below saturation density, but
particularly at high density, where very few relevant experimental data exist.
We discuss observables which could yield information on this question, in
particular, proton-neutron flow differences, and the production of pions and
kaons in relativistic heavy ion collisions. For the meson production we
investigate particularly ratios of the corresponding isospin partners
$\pi^-/\pi^+$ and $K^0/K^+$, where we find that the kaons are an interesting
probe to the symmetry energy. In this case we also discuss the influnece of
various choices for the kaon potentials or in-medium effective masses.
