  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.
