Taking full advantage of electron spin in spin-electronics will eventually require an intimate integration of ferromagnetic and semiconductor materials. While device concepts are emerging, the understanding of spin transport in such hybrid ferromagnet/semiconductor structures is still at its infancy. We have focused on transport of non-equilibrium, hot-electron spins, for which spin currents can be controlled and manipulated via the electron energy and momentum. A particularly useful device for that purpose is the spin-valve transistor@footnote 1@, consisting of a metallic spin-valve base, sandwiched between a semiconductor emitter and collector. Using the spin-valve transistor, we address the relative importance of interface, volume and thermal scattering of hot electron spins, and present new insight into the sources of spin-asymmetry in hot-electron transport.@footnote 2,3@ From an application point of view, enhancing the output current of the transistor is desired. We demonstrate several routes to enhance the transfer ratio, culminating in an overall improvement by two orders of magnitude while preserving the low-field magnetic response above 200% at room temperature.@footnote 4@ We also present transport in novel structures such as the magnetic tunnel transistor and the hot-electron spin-filter, and demonstrate that the latter allows room temperature injection of almost fully spin-polarized electrons into semiconductors. @FootnoteText@ @footnote 1@ R. Jansen et al., J. Appl. Phys. 89, 7431 (2001). @footnote 2@ R. Jansen et al., Phys. Rev. Lett. 85, 3277 (2000). @footnote 3@ R. Vlutters et al., Phys. Rev. Lett. 88, 027202 (2002). @footnote 4@ O.M.J. van 't Erve et al., Appl. Phys. Lett. 80, to appear 20 may 2002.