  Single molecule magnets straddle the classical and quantum mechanical worlds,
displaying many fascinating phenomena. They may have important technological
applications in information storage and quantum computation. We review the
physical properties of two prototypical molecular nanomagnets, Mn_12-acetate
and Fe_8: each behaves as a rigid, spin-10 object, and exhibits tunneling
between up and down directions.
  As temperature is lowered, the spin reversal process evolves from thermal
activation to pure quantum tunneling. At low temperatures, magnetic avalanches
occur in which the magnetization of an entire sample rapidly reverses. We
discuss the important role that symmetry-breaking fields play in driving
tunneling and in producing Berry-phase interference. Recent experimental
advances indicate that quantum coherence can be maintained on time scales
sufficient to allow a meaningful number of quantum computing operations to be
performed. Efforts are underway to create monolayers and to address and
manipulate individual molecules.
