The relaxation dynamics of photoexcited CdSe nanoparticles (NPs) were studied with femtosecond pump−probe spectroscopy in the spectral range from 450 nm to 5 μm. Thus, the intraband relaxation of the electron and the hole were monitored with femtosecond time resolution. The transition from delocalized electronic to localized defect states and the slower relaxation through the trapping sites (trap hopping mechanism) were followed by time-resolved emission on time scales from picoseconds to milliseconds in the spectral range from the visible into the mid-infrared. The spectral dynamics in the visible, near-infrared (NIR), and infrared (IR) range give a mechanistic picture about the relaxation pathway of the excited charge carriers in CdSe NPs. In addition, by using electron and hole quenchers to the photoexcited nanoparticles, we could assign the observed dynamics to the electron or the hole. In the visible range, bleach features (negative signals in the differential transient absorption measurements) were ...