  I derive a general set of boundary conditions for quasiclassical transport
theory of metals and superconductors that is valid for equilibrium and
non-equilibrium situations and includes multi-band systems, weakly and strongly
spin-polarized systems, and disordered systems. The formulation is in terms of
the normal state scattering matrix. Various special cases for boundary
conditions are known in the literature, that are however limited to either
equilibrium situations or single band systems. The present formulation unifies
and extends all these results. In this paper I will present the general theory
in terms of coherence functions and distribution functions and demonstrate its
use by applying it to the problem of spin-active interfaces in superconducting
devices and the case of superconductor/half-metal interface scattering.
