  Phase transitions which occur at zero temperature when some non-thermal
parameter like pressure, chemical composition or magnetic field is changed are
called quantum phase transitions. They are caused by quantum fluctuations which
are a consequence of Heisenberg's uncertainty principle. These lecture notes
give a pedagogical introduction to quantum phase transitions. After collecting
a few basic facts about phase transitions and critical behavior we discuss the
importance of quantum mechanics and the relation between quantum and classical
transitions as well as their experimental relevance. As a primary example we
then consider the Ising model in a transverse field. We also briefly discuss
quantum phase transitions in itinerant electron systems and their connection to
non-Fermi liquid behavior.
