This text introduces the subject of relativistic high-energy heavy-ion collisions and in particular, the subject of the quark–gluon plasma (QGP). Starting with a conceptual basis for QGP formation in heavy-ion collisions, the author then proceeds to provide a more rigorous foundation by introducing gauge theory, QCD and lattice QCD. These topics are introduced briefly but with sufficient coverage that the reader can comprehend their applications in heavy-ion collisions. Two-particle correlation (Hanbury-Brown-Twiss) method and recent advances in hydrodynamical modelling, including event-by-event hydrodynamics are also discussed, bringing the coverage up to the leading areas of current research.