New applications of magnetism emerge in "spin electronics" in which the electric currents are governed by the spin state of the electron, and in which ferromagnetic metals act as the sources and the detectors of the spin polarization P. There are two types of ferromagnetic metal, one with positiv P at the Fermienergy E_F and the other with negative P. Crucial to the development of spin electronics is the understanding of the motion and attenuation of the spin polarization vector P in transport of the electrons through solids.We discuss 3 different experiments in which this may be studied: (i) low energy spin polarized electron spectroscopy, (ii) two photon-photoemission with pump-probe femtosecond laser pulses, and (iii) transmission experiments on trilayers with low energy spin polarized electron beams. We show that electron/electron scattering is the main agent producing spin filering in 3d-ferromagnetic metals due to inelastic electron scattering into the holes of the 3d-shell. This causes a rotation of P into the direction of the magnetization M. P also performs a fast precession around the direction of M. This precession yields unique information on the exchange splitting in energy states far above E_F and may be applied to imaging of the magnetization in analogy to magneto-optic phenomena. Further applications include magnetization reversal through injection of spin polarized currents. Inelastic electron scattering as well as the torque exercised by the exchange interaction give rise to new types of magnetic relaxation.