  These are notes for lectures delivered at the NATO ASI on Dynamics in Leiden,
The Netherlands, in July 1998. The main concepts relating to quantum phase
transitions are explained, using the paramagnet-to-ferromagnet transition of
itinerant electrons as the primary example. Some aspects of metal-insulator
transitions are also briefly discussed. The exposition is strictly pedagogical
in nature, with no ambitions with respect to completeness or going into
technical details. The goal of the lectures is to provide a bridge between
textbooks on classical critical phenomena and the current literature on quantum
phase transitions. Some familiarity with the concepts of classical phase
transitions is helpful, but not absolutely necessary.
