Relativistic Heavy Ion Collisions provide a way to study properties of matter interacting via strong force in the laboratory conditions. The World-wide effort studying such collisions includes several international collaborations analyzing data from several experiments such as ALICE, ATLAS, CMS operating at the Large Hadron Collider at CERN and PHENIX and STAR taking data at the Relativistic Heavy Ion Collider at BNL, as well as experiments previously worked at the SPS, AGS and SIS accelerators. Several new projects are under development in different countries.

The strongly interacting matter created in such collisions exists in a state called strongly coupled Quark-Gluon Plasma, where the main constituents are not hardons but quarks and gluons. The same state of matter is believed was the state of the Universe a fraction of a second after the Big Bang. In the present Universe it might exist inside the neutron stars.

The Quark-Gluon Plasma exhibits strong collective behavior and imposes a gigantic, compared to ordinary matter, energy loss on strong force carriers propagating through it. A review of the modern state of the field will be given in the talk.