  We provide an overview of the present understanding of the transition from
hadrons to a quark-gluon plasma, its signatures, and the experimental results
so far. We discuss results of numerical simulations of the lattice gauge theory
and critically evaluate the various observables that have been proposed as
signatures of the QCD phase transition. We place the existing data from
relativistic heavy ion experiments at the Brookhaven AGS and CERN SPS into
perspective and provide an overview of the techniques and strategies that will
be employed in the search for the quark-gluon plasma at heavy ion colliders,
such as RHIC and the LHC.
