  The BUU transport model is applied to study strangeness and pion production
in nucleus-nucleus collisions. Starting from proton induced reactions, we
further investigate Si+Au, Au+Au and Pb+Pb collisions in the energy range
between 2 and 40 A$\cdot$GeV and compare with data and with other transport
calculations. The $q \bar q$ annihilation, or resonance, channel simulated by
the string model in meson-nucleon collisions at $\sqrt{s} > 2$ GeV is
introduced. The importance of this channel for a good description of the
proton-nucleus data on $K^+$ production is demonstrated. We, furthermore, show
that the meson-meson collisions contribute significantly to the $K \bar K$
production in heavy-ion collisions above 5 A GeV and improve an agreement with
data on the $K^+/\pi^+$ ratio. Finally, we study the influence of the in-medium
modifications of the FRITIOF model on the pion and kaon production.
