Quark-Gluon Plasma introduces the primordial matter, composed of two types of elementary particles, created at the time of the Big Bang. During the evolution of the universe, Quark-Gluon Plasma (QGP) undergoes a transition to hadronic matter governed by quantum chromodynamics, the law of strong interactions. After an introduction to gauge theories, various aspects of quantum chromodynamic phase transitions are illustrated in a self-contained manner. The cosmological approach and renormalization group are discussed, as well as the cosmological and astrophysical implications of QGP, on the basis of Einstein's equations. Recent developments towards the formation of QGP in ultrarelativistic heavy ion collisions are also presented in detail. This text is suitable as an introduction for graduate students, as well as providing a valuable reference for researchers already working in this and related fields. It includes eight appendices and over a hundred exercises.