This book introduces quark gluon-plasma (QGP) as a primordial matter composed of quarks and gluons, created at the time of the "Big Bang". After a pedagogical 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 covered, 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.