Conduction electrons passing through a non-magnetic layer may carry spinpolarisation and hence influence the relative magnetisations of the ferromagnetic metal layers in a standard trilayer spin valve configuration. Away from equilibrium, where a Rudermann-Kittel type of exchange interaction exists between the ferromagnetic spins,a spin current gives rise to a further strong NEXI(non-equilibrium exchange interaction).This usually dominates and can cause changes in the magnetic configuration of the layers, involving their domain structures and complete switching. The theory of these effects, including the dynamics of such switching will be reviewed.