Recently reported theoretical and experimental developments in quark-gluon plasma are reviewed. After an introduction to the quantum chromodynamics (QCD) that governs the dymanics of quarks and gluons, we describe the thermal history of the early universe and the space-time history of relativistic heavy-ion collisions. Some results of an ongoing experiment at the RHIC (Relativisitic Heavy Ion Collider) at Brookhaven National Laboratory are presented together with remarks on future study.