Most applications in electronics are based on manipulation of the electron charge. Currently, there is also a lot of research into the possibility to make use of the electron spin, for example in magnetoelectronics. This field of research studies hybrid systems consisting of ferromagnetic metals, paramagnetic metals and insulators. In this thesis we present a theoretical description of a number of physical structures whose operation is based on the spin-valve principle. A spin valve consists of two ferromagnetic elements with a non-magnetic material in between. The electrical resistance of a spin valve can be varied by changing the angle between the magnetization directions of the two ferromagnets. We are especially interested in effects of the interaction between electrons on electronic transport in spin-valve structures. As the experimentally studied structures are getting smaller, interactions play an increasingly important role.