Recent experiments on magnetic nanostructured materials revealed new phenomena associated with the interplay of ferromagnetism and discrete charging effects. A typical example is a ferromagnetic single-electron transistor, i.e., a small grain or quantum dot (QD) coupled by tunnel junctions to ferromagnetic electrodes. We investigate the interplay of charge and spin degrees of freedom in these systems in the sequential tunneling, cotunneling and strong coupling regimes. The description is formulated in a two-dimensional space of charge and spin states, and allows us to calculate electric and spin currents, spin and charge accumulation and fluctuations, as well as the tunnel magnetoresistance. For QDs, we find a Kondo resonance at low temperatures, which significantly depends on the spin polarization of conduction electrons in the leads.

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