  Many materials contain extended defects of nanosize scale, such as
dislocations, cracks, pores, polymorphic inclusions, and other embryos of
competing phases. When one is interested not in the precise internal structure
of a sample with such defects, but in its overall properties as a whole, one
needs a statistical picture giving a spatially averaged description. In this
chapter, an approach is presented for a statistical description of materials
with extended nanosize defects. A method is developed allowing for the
reduction of the problem to the consideration of a set of system replicas
representing homogeneous materials characterized by effective renormalized
Hamiltonians. This is achieved by defining a procedure of averaging over
heterophase configurations. The method is illustrated by a lattice model with
randomly distributed regions of disorder.
