  The properties of $\pi, \eta, K^+$ and $K^-$ mesons are studied in nuclear
reactions from SIS to SPS energies within the covariant transport approach HSD
in comparison to the experimental data. Whereas the pion, $\eta$ and kaon
abundancies and spectra indicate little or vanishing selfe-nergies for these
mesons in the medium, antikaons (as well as antiprotons) 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 well
described by the hadronic approach without incorporating any parton degrees of
freedom.
