  In this Thesis we study the quantum to classical transition process in the
context of quantum mechanics and quantum field theory. We shall analyze the
effects that general environments, namely ohmic and non-ohmic, at zero and high
temperature induce over a quantum Brownian particle. We state that the
evolution of the system can be summarized in terms of two main environmental
induced physical phenomena: decoherence and energy activation. In this Thesis,
we shall show that the latter is a post-decoherence phenomenon. As the energy
is an observable, the excitation process can be consider a direct indicator of
the system-environment entanglement particularly useful at zero temperature.
  From other point of view, we shall study different attempts to show the
decoherence process in double-slit-like experiments both for charged particles
(electrons) and for neutral particles with permanent dipole moments. In this
context, we shall show that the interaction between the particles and
time-dependent fields induces a time-varying Aharonov phase. In this context,
we shall apply our results to a real matter wave interferometry experiment. We
shall also show under which general conditions the geometry phase of a quantum
open system can be observed.Finally, we shall study the decoherence process
during a quantum phase transition. In this framework, we shall show that it can
be phrased easily in terms of the decoherence functional, without having to use
the master equation. To demonstrate this, we shall consider the decohering
effects due to the displacement of domain boundaries, with implications for the
displacement of defects, in general. We shall see that decoherence arises so
quickly in this event, that it is negligible in comparison to decoherence due
to field fluctuations in the way defined in previous papers.
