  Kaon production, in particular $K^+$ production in heavy ion collisions at
intermediate energies is discussed. Main emphasis is put on the question if
subthreshold $K^+$ production can serve as a suitable tool to test the high
density phase of such reactions and to deliver information on the high density
behavior of the nuclear equation of state. It is shown that the $K^+$
excitation function in heavy ($Au+Au$) over light ($C+C$) systems provides a
robust observable which, by comparison to data, strongly favors a soft equation
of state. A second question of interest is the existence of an in-medium kaon
potential as predicted by effective chiral Lagrangiens. Here it is argued that
transport calculations support this scenario with, in the meantime, a
significant level of consistency.
