  Based on the technique of quasiclassical Green's functions, we construct a
theoretical framework for describing heterostructures consisting of
superconductors and/or spin-polarized materials. The necessary boundary
conditions at the interfaces separating different metals are formulated in
terms of hopping amplitudes in a t-matrix approximation. The theory is
applicable for an interface with arbitrary transmission and exhibiting
scattering with arbitrary spin dependence. Also, it can be used in describing
both ballistic and diffusive systems. We establish the connection between the
standard scattering-matrix approach and the existing boundary conditions, and
demonstrate the advantages offered by the t-matrix description.
