  The properties of $K^+$ and $K^-$ mesons are studied in nuclear reactions
from SIS to RHIC energies within the covariant transport approach HSD in
comparison to the experimental data whenever available. Whereas kaon
abundancies and spectra indicate little repulsive or vanishing selfenergies in
the medium, antikaons are found to experience strong attractive potentials in
nucleus-nucleus collisions at SIS energies. However, even when including these
potentials the $K^+$ and $K^-$ spectra at AGS energies are noticeably
underestimated showing an experimental excess of strangeness that points
towards a nonhadronic phase in these reactions. On the other hand the $K^+,
K^-$ production at SPS energies is again well described by the HSD approach
based on quark, diquark, string and hadronic degress of freedom. At RHIC
energies hadronic rescatterings are found to enhance the strangeness yield from
the partonic phase quite substantially.
