  We study spin dependent phonomena in superconducting junctions in both
ballistic and diffusive regimes. For ballistic junctions we study both
ferromagnet / $s$- and d-wave superconductor junctions and two dimensional
electron gas / s-wave superconductor junctions with Rashba spin-orbit coupling.
It is shown that the exchange field alway suppresses the conductance while the
Rashba spin-orbit coupling can enhance it. In the latter part of the article we
study the diffusive ferromagnet / insulator / $s$- and d-wave superconductor
junctions, where the proximity effect can be enhanced by the exchange field in
contrast to common belief. It is shown that the resonant proximity effect
originating from the exchange field strongly influences the tunneling
conductance and density of states.
