  The aim of this paper is to review a new perspective about decoherence,
according to which formalisms originally devised to deal just with closed or
open systems can be subsumed under a closed-system approach that generalizes
the traditional account of the phenomenon. This new viewpoint dissolves certain
conceptual difficulties of the orthodox open-system approach but, at the same
time, shows that the openness of the quantum system is not the essential
ingredient for decoherence, as commonly claimed. Moreover, when the behavior of
a decoherent system is described from a closed-system perspective, the account
of decoherence turns out to be more general than that supplied by the
open-system approach, and the quantum-to-classical transition defines
unequivocally the realm of classicality by identifying the observables with
classical-like behavior.
