The physics of high baryon densities

I review the physics of strongly interacting matter at high (and not so high) baryon densities, as probed in nuclear collisions. Recent lattice results on the critical end point and the possibility to find it in experiment by searching for event-by-event fluctuations are discussed. Furthermore, I summarize the theoretical ideas on the in-medium properties of hadrons at non-zero baryon densities, in particular for kaons, D mesons and light vector mesons. Finally, I briefly discuss the role of baryon density for strangeness production in heavy-ion collisions and the possibility to obtain experimental information on hadron properties in cold nuclear matter from hadronic and electromagnetic reactions on nuclei.