Relativistic heavy ion reactions

The present status of the study of central collisions of relativistic heavy ions is presented. The phenomenology is described and evidence is presented for a source of nucleons from a central region caused by the overlapping densities of the target and projectile. Some of the current theoretical approaches are described including the nuclear fireball model. It appears that there is a quasiequilibrated region at high temperature, but the signature for the expected effects of high density is not yet clear, and therefore, experimentally, the effects of high density have not been identified. (auth.)