  We study the effect of spin injection into $s-$ and $d-$wave superconductors,
with an emphasis on the interplay between boundary and bulk spin transport
properties. The quantities of interest include the amount of non-equilibrium
magnetization ($m$), as well as the induced spin-dependent current ($I_s$) and
boundary voltage ($V_s$). In general, the Andreev reflection makes each of the
three quantities depend on a different combination of the boundary and bulk
contributions. The situation simplifies either for half-metallic ferromagnets
or in the strong barrier limit, where both $V_s$ and $m$ depend solely on the
bulk spin transport/relaxation properties. The implications of our results for
the on-going spin injection experiments in high $T_c$ cuprates are discussed.
