Physics and Signatures of the Quark - Gluon Plasma

This is a critical review of the various observables that have been proposed to signal the change from dense hadronic matter to a quark-gluon plasma at high temperature or baryon density. I discuss current models of quark-gluon plasma formation in relativistic heavy ion collisions and analyze the virtues and ambiguities of various signatures. [The author especially invites comments by interested readers concerning possible inaccuracies or omissions before this review appears in print.]