Investigation of the time evolution of extremely nonequilibrium charge carriers photoexcited into a semiconductor by a femtosecond laser gives valuable information about various scattering processes in the semiconductor. We describe here (i) an excite-and-probe experiment in GaAs quantum wells which measures the time evolution of the athermal distribution produced by the laser and thus gives information about carrier-carrier scattering processes and (ii) a luminescence experiment in GaAs and GaAs quantum wells which gives information about intervalley scattering of electrons in the conduction band.