  Zero-temperature or quantum phase transitions in itinerant electronic systems
both with and without quenched disordered are discussed. Phase transitions
considered include, the ferromagnetic transition, the antiferromagnetic
transition, the superconductor-metal transition, and various metal-insulator
transitions. Emphasis is placed on how to determine the universal properties
that characterize these quantum phase transitions. For the first three of the
phase transitions listed above, one of the main physical ideas established is
that in zero-temperature systems there are soft or slow modes that exist in
addition to the soft order parameter fluctuations, and that these modes can
couple to the critical modes. These extra soft modes are shown to have a
profound effect on the quantum critical properties. For quantum phase
transitions involving zero wavenumber order parameters, i.e., the ferromagnetic
and superconductor-metal transitions, these extra modes effectively lead to
long-ranged effective interactions between order parameter fluctuations, which
in turn lead to exactly soluble critical behaviors. For the antiferromagnetic
case, we argue that while in low enough dimensions disorder fluctuation effects
tend to destroy long-range order, quantum fluctuations counteract this effect
and in some parameter regions manage to re-establish antiferromagnetic
long-range order. For the metal-insulator transition, some recent new ideas are
reviewed. In particular, it is pointed out that for interacting disordered
electrons, one expects that in high dimensions the metal-insulator transition
is related to the phase transition that occurs in random-field magnets in high
dimensions. If the analogy also holds in three dimensions this suggests that
the metal-insulator transition might have glassy characteristics.
