Because nucleons are bound states of quarks, nuclear matter should at sufficiently high density become a quark plasma. Assuming quantum chromodynamics as the basic theory of nuclear forces, it is possible to derive strong-interaction thermodynamics and study the nature of this transition. Predictions of thermodynamic variables are obtained by computer simulation; they provide the first theoretical information on the phase structure of strongly interacting matter. The experimental search for the quark plasma is beginning this year at Brookhaven and at CERN. Energetic nuclear collisions could produce sufficiently dense and long-lived strongly interacting systems to allow a first look at the predicted new state of matter.