  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.]
