  The recent past witnesses the growing interdependence between the physics of
hadrons, the physics of relativistic heavy-ion collisions, and the physics of
compact objects in astrophysics. A notable example is the kaon which plays
special roles in all the three fields. In this talk, we first review the
various theoretical investigations of kaon properties in nuclear medium. We
then present a detailed transport model study of kaon production and flow in
heavy-ion collisions at SIS energies. Finally, We discuss the effects of the
kaon in-medium properties extracted from heavy-ion data on neutron star
properties, especially on the lowering of maximum mass of neutron stars with
the onset of kaon condensation around $3\rho_0$.
