  The low-energy physics of systems coupled to their surroundings is understood
by truncating to effective Hamiltonians; these tend to reduce to a few
canonical forms, involving coupling to "baths" of oscillators or spins. The
method for doing this is demonstrated using examples from magnetism,
superconductivity, and measurement theory, as is the way one then solves for
the low-energy dynamics. Finally, detailed application is given to the exciting
recent Quantum relaxation and tunneling work in naomagnets.
