  >A review of meson emission in heavy ion collisions at incident energies from
SIS up to collider energies is presented. A statistical model assuming chemical
equilibrium and local strangeness conservation (i.e. strangeness conservation
per collision) explains most of the observed features.
  Emphasis is put onto the study of $K^+$ and $K^-$ emission at low incident
energies. In the framework of this statistical model it is shown that the
experimentally observed equality of $K^+$ and $K^-$ rates at
``threshold-corrected'' energies $\sqrt{s} - \sqrt{s_{th}}$ is due to a
crossing of two excitation functions. Furthermore, the independence of the
$K^+/K^-$ ratio on the number of participating nucleons observed between SIS
and RHIC is consistent with this model.
  It is demonstrated that the $K^-$ production at SIS energies occurs
predominantly via strangeness exchange and that this channel is approaching
chemical equilibrium. The observed maximum in the $K^+/\pi^+$ excitation
function is also seen in the ratio of strange to non-strange particle
production. The appearance of this maximum around 30 $A\cdot$GeV is due to the
energy dependence of the chemical freeze-out parameters $T$ and $\mu_B$.
