Heavy-ion collisions provide the unique possibility to study baryonic matter well above saturation density. The conditions inside the dense reaction zone and the in-medium properties of hadrons can be explored by measuring the particles created in such collisions. In particular, the production of strange mesons at beam energies below or close to their respective threshold in binary nucleon-nucleon collisions is well suited for these studies. The production of K+ and of K- mesons as well as of pions in heavy-ion collisions at beam energies of 1 to 2 AGeV has systematically been investigated with the Kaon Spectrometer KaoS at the heavy-ion synchrotron SIS at the GSI in Darmstadt, Germany. The ratio of the K+ production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. The K-/K+ ratio is found to be nearly constant as a function of the collision centrality and can be explained by the dominance of strangeness exchange. On the other hand the spectral slopes and the polar emission patterns are different for K- and for K+. Furthermore the azimuthal distribution of the particle emission has been investigated. K+ mesons and pions are emitted prefentially perpendicular to the reaction plane as well in Au+Au as in Ni+Ni collisions. In contrast for K- mesons in Ni+Ni reactions an in-plane flow was observed for the first time at these incident enegies.
