  We summarize what we have learnt about the kaon production in nucleus-nucleus
collisions in the last decade. We will address three questions: a) Is the $K^+$
production sensitive to the nuclear equation of state? b) How can it happen
that at the same excess energy the same number of $K^+$ and $K^-$ are produced
in heavy ion collisions although the elementary cross section in pp collisions
differs by orders of magnitudes? and c) Why kaons don't flow?
