  An approach to the description of subdynamics inside non-relativistic quantum
field theory is presented, in which the notions of relevant observable, time
scale and complete positivity of the time evolution are stressed. A scattering
theory derivation of the subdynamics of a microsystem interacting through
collisions with a macrosystem is given, leading to a master-equation expressed
in terms of the operator-valued dynamic structure factor, a two-point
correlation function which compactly takes the statistical mechanics properties
of the macrosystem into account. For the case of a free quantum gas the dynamic
structure factor can be exactly calculated and in the long wavelength limit a
Fokker-Planck equation for the description of quantum dissipation and in
particular quantum Brownian motion is obtained, where peculiar corrections due
to quantum statistics can be put into evidence.
