QCD In Extreme Conditions

Recently we have made considerable progress in our understanding of the behavior of QCD in extreme conditions of high temperature or large baryon number density. Among the highlights are the prediction of a well-characterized true critical point, and the discovery that the ground state of three-flavor QCD at asymptotically high densities exhibits color-flavor locking. The critical point occurs at the unique temperature and density where a sharp distinction between an ionized plasma of quarks and gluons and the hadronic phase first appears. It appears to be accessible both to numerical and to laboratory experiments. Color-flavor locking provides a calculable, weak-coupling realization of confinement and chiral symmetry breaking. It also provides a microscopic realization of Han-Nambu charge assignments for quark quasiparticles, and of Yang-Mills theory for the physical vector mesons. Here I provide a self-contained introduction to these developments.