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The Andreev reflection mechanism at Superconducting: Normal metal interfaces reverses both the sign of the charge and of the spin of the incoming particle from the N side. In an ideal contact, the conductance at small bias is increased by a factor of 2 compared to that in the normal state. In the presence of a barrier at the interface, the probability of Andreev reflections is diminished, but remains finite, giving a subgap conductance. Spin polarization in the normal metal also reduces the probability of Andreev reflections, which are completely quenched for a full polarization. In that case, the resistance of the contact is infinite, even in the absence of any barrier. Yet, if a second contact is made within a distance of the order of the coherence length, with a metal having an opposite polarization, the full Andreev increased conductance can be recovered when the two contacts are connected in series. Such a device thus has a strong magnetoresistance upon reversal of the magnetization of one of the two contacts. Different consequences of these crossed Andreev reflections are discussed.