QCD and heavy ions

This short paper is an attempt to describe a theorist's view of the goals of relativistic heavy ion program which has just entered the collider era. These goals are centered around understanding the properties and the critical behavior of Quantum Chromo-Dynamics (QCD) in the non-linear regime of high color field strength and high parton density. Some of the current theoretical challenges are highlighted, and the place of heavy ion research in the broader context of modern particle and nuclear physics is discussed.