Knowledge of excited electronic states in semiconductor quantum dots (QDs) is of fundamental scientific interest and is important for application in lasers, optical detectors, LEDs, solar cells, photocatalysis, biomedical imaging, photodynamic therapy etc. In the past few years, carrier multiplication (CM) in QDs has received particular attention, due to promising prospects for exploitation in highly efficient solar cells, photodetectors and possibly photocatalysis. CM can occur when absorption of a high-energy photon leads to production of an excited electron or a hole with an excess energy that exceeds the QD band gap. CM involves transfer of (part of) the excess energy of the excited electron or hole to one or more valence electrons that also become excited across the band gap via a process denoted as impact ionization. In this way absorption of a single photon can lead to excitation of two or more electrons. This thesis describes studies of factors affecting the efficiency of CM in QDs based on PbSe. Ultrafast time-resolved optical pump and probe spectroscopy is used to characterize the nature of photoexcited states, the efficiency of CM, hot exciton cooling and Auger recombination of multiexcitons. The occurrence of CM has been reported by several research groups for QDs consisting of PbX (X = S, Se, Te), CdSe, InAs, and Si. However, in some other studies CM was not observed for CdSe, CdTe, and InAs QDs, thus raising legitimate doubts concerning the occurrence of CM in the other materials. In the work of chapter 2 conclusive evidence is given for the occurrence of CM in PbSe QDs. Possible artifacts due multi-photon absorption and charge trapping are excluded. It is shown that for higher exciton multiplicity a correct determination of the CM efficiency requires spectral integration over the photobleach feature. The CM efficiency of ?CM = 1.7 obtained at a photon energy of 4.8 times the band gap is close to results that have appeared in the literature more recently. Chapter 3 describes studies of the dynamics of hot excitons in PbSe QDs, PbSe/PbS core/shell QDs, and PbSe/PbSexS1-x core/alloyed-shell QDs. The ground state optical absorption exhibits a red-shift on introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations, this can be attributed to electron delocalization into the shell. Remarkably, the CM efficiency, the hot exciton cooling rate, and the Auger recombination rate of multiexcitons are similar for PbSe core-only QDs and core/shell QDs with the same core size and varying shell thickness, despite the marked variations in the density of states evidenced by the changes in optical spectra. It is concluded that different effects that may serve to speed up or slow down exciton dynamics, such as variations in density of states, shell-induced asymmetry in the band structure and hot exciton cooling counteract one another. The second transition in the ground state optical absorption spectrum of PbSe QDs is arguably the most discussed optical transition in semiconductor QDs. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1Pe1Ph as well as the 1Sh,e1Pe,h transitions. The studies described in chapter 4 show that the strength of the second optical transition in the absorption spectrum of PbSe QDs is not affected by the presence of 1Sh1Se excitons, even if four of those excitons are introduced. Hence, the second optical transition involves neither 1Se nor 1Sh states. This suggests that it is the 1Ph1Pe transition that gives rise to the second peak in the absorption spectrum of PbSe QDs. The transitions causing extinction in the energy region between the 1Sh1Se and 1Ph1Pe transitions was investigated, as described in chapter 5. The ultrafast transient absorption data indicate that the extinction in this region is not due to Rayleigh scattering, nor to local field effects, but to the formally forbidden 1Ph1Se and 1Sh1Pe transitions. These optical transitions can become allowed, due to deviations of the QD shape from ideal spherical symmetry. For applications, it is essential that multiple charges are extracted from multiexcitons generated within a QD, prior to decay by Auger recombination. Therefore the optical properties and decay kinetics of multiexcitons were studied. The results are presented in chapter 6. The first and second optical transitions in the ground state absorption spectrum of PbSe QDs are strongly shifted to the red as the number of 1Sh1Se spectator excitons increases. These red-shifts can be attributed to Coulomb interactions. The lifetimes before Auger decay of 1Sh1Se multiexcitons were determined. The population decay for 6.8 nm PbSe QDs could be described by assuming the Auger recombination rate to increase exponentially with the number of excitons in a QD. For smaller QDs, the exponential and another 3-charge interaction model reproduce the experimental data equally well. Further studies are needed to unravel the effects of QD size on the dependence of Auger recombination on the number of excitons.