One of the important issues of contemporary physics is the understanding of strong interactions and in
particular the study of the properties of strongly interacting matter in equilibrium. It is commonly expected that the matter at very high densities may exist in a state of quasi-free quarks and gluons, the quark-gluon plasma (QGP). Does a QGP exist in nature? What does the transition from a low-density state of strongly interacting matter, in which quarks and gluons are confined in hadrons, to QGP look like? Is it similar to the transition from liquid water to water vapor: the first order transition line ending by the second order critical point and followed by the cross over? How does the transition between a non-equilibrium system created in inelastic proton-proton interactions and the equilibrium one in central heavy ion collisions look like?
In this talk I will review experimental results as well as running and future experimental programmes on heavy ion collisions which address these critical questions.