For a system consisting of a random medium with roughboundaries, the governing (Bethe-Salpeter) equation forboundary-value transport problems can be written in a formsuch that the medium and the boundaries are treatedon anequal footing. This enables several expressions for thesolution to be obtained by interchanging the roles of themedium and the boundaries, thus allowing the most convenientone to be selected according to the specific situation andthe information required. This book presents a unifiedtheory based on the Bethe-Salpeter equation with particularattention being paid to: boundary-value problems oftransport, layer problems, a fixed scatterer imbedded in abounded random medium, construction of an optical scatteringmatrix for a complete system, and optical wave propagationin a turbulent medium. The last topic is treated in terms offirst moment equations combined with the cluster expansionand, second, the two-scale method based on the Lagrangevariational principle.