IntroductionUltrafast dynamics at surfaces and in thin films can be unraveled by femtosecond laser experiments. In particular, two-photon photoemission (2PPE) allows us to study the dynamics of photoexcited electrons directly in the time domain [1,2]. Decay processes determine the lifetime of photoexcited carriers and are by nature inelastic. In a metal, an electron photoexcited to states above the Fermi level will decay into empty states below. To conserve energy and momentum, for example, a second electron out of the Fermi sea is excited. In contrast, in a quasielastic scattering process, only the momentum of the photoexcited carriers is changed. Momentum scattering usually does not alter the population, but is important in transport processes. Moreover, quasielastic scattering causes dephasing, that is, polarization decay upon optical excitation. Besides understanding these fundamental aspects [2][3][4][5][6][7][8][9][10][11][12], the driving power for this active research field are the impact of excited, hot electrons and holes in chemical reactions [13][14][15][16] as well as the importance of electron transport through semiconducting and ferromagnetic devices [1,17]. A number of excellent review articles considering these topics have been published [1,2,18].In this chapter, we describe some aspects of spin-dependent decay and dephasing processes at surfaces of 3d ferromagnets. Layered magnetic structures are ubiquitous in todays technical application, and transport processes in these devices will be affected by spin-dependent lifetimes of carriers and spin-dependent scattering at surfaces and interfaces.In a ferromagnet, the band structure is exchange split with a spin-dependent density of states that leads to unequal occupation numbers for majority and minority spin carriers. Thus, relaxation processes become spin dependent and, in general, minority spin electrons have a shorter lifetime than their majority spin counterparts [10,17,[19][20][21][22]. In addition, spin waves can be excited upon electron scattering. Low-energy magnons will cause dephasing while high-energy magnons contribute to inelastic decay of photoexcited carriers. We note that in the ground state of the Dynamics at Solid State Surface and Interfaces Vol.