  We present a theory of electron, electromagnetic, and elastic wave
propagation in systems consisting of non-overlapping scatterers in a host
medium. The theory provides a framework for a unified description of wave
propagation in three-dimensional periodic structures, finite slabs of layered
structures, and systems with impurities: isolated impurities, impurity
aggregates, or randomly distributed impurities. We point out the similarities
and differences between the different cases considered, and discuss the
numerical implementation of the formalism.
