  The main goal of the present paper is to convince that it is feasible to
construct a `periodic orbit theory' of localization by extending the idea of
classical action correlations. This possibility had been questioned by many
researchers in the field of `Quantum Chaos'. Starting from the semiclassical
trace formula, we formulate a quantal-classical duality relation that connects
the spectral properties of the quantal spectrum to the statistical properties
of lengths of periodic orbits. By identifying the classical correlation scale
it is possible to extend the semiclassical theory of spectral statistics, in
case of a complex systems, beyond the limitations that are implied by the
diagonal approximation. We discuss the quantal dynamics of a particle in a
disordered system. The various regimes are defined in terms of time-disorder
`phase diagram'. As expected, the breaktime may be `disorder limited' rather
than `volume limited', leading to localization if it is shorter than the
ergodic time. Qualitative agreement with scaling theory of localization in one
to three dimensions is demonstrated.
