  I review the decoherent (or consistent) histories approach to quantum
mechanics, due to Griffiths, to Gell-Mann and Hartle, and to Omnes. This is an
approach to standard quantum theory specifically designed to apply to genuinely
closed systems, up to and including the entire universe. It does not depend on
an assumed separation of classical and quantum domains, on notions of
measurement, or on collapse of the wave function. Its primary aim is to find
sets of histories for closed systems exhibiting negligble interference, and
therefore, to which probabilities may be assigned. Such sets of histories are
called consistent or decoherent, and may be manipulated according to the rules
of ordinary (Boolean) logic. The approach provides a framework from which one
may predict the emergence of an approximately classical domain for macroscopic
systems, together with the conventional Copenhagen quantum mechanics for
microscropic subsystems. In the special case in which the total closed system
naturally separates into a distinguished subsystem coupled to an environment,
the decoherent histories approach is closed related to the quantum state
diffusion approach of Gisin and Percival.
