This bookis based on some of the lectures duringthe Paci?c Institute of
Theoretical Physics (PITP) summer school on "Quantum Magnetism", held
during June 2006 in Les Houches, in the French Alps. The school was
funded jointly by NATO, the CNRS, and PITP, and entirely organized by
PITP. Magnetism is a somewhat peculiar research ?eld. It clearly has a
quant- mechanical basis - the microscopic exchange interactions arise
entirely from the exclusion principle, in conjunction with repulsive
interactions between electrons. And yet until recently the vast majority
of magnetism researchersand users of m- netic phenomena around the world
paid no attention to these quantum-mechanical roots. Thus, e.g., the
huge ($400 billion per annum) industry which manufactures hard discs,
and other components in the information technology sector, depends
entirely on room-temperature properties of magnets - yet at the
macroscopic or mesoscopic scales of interest to this industry,
room-temperature magnets behave entirely classically.