Due to their size-dependent properties, high photoluminescence quantum yield and relatively cheap solution-based processing, colloidal quantum dots (QDs) are of great interest for application in optoelectronic devices. However, the efficiency of these devices is often limited by the presence of trap states: localized electronic states that lead to energy levels in the bandgap. Although much research has been geared to passivating (i.e., removing) these trap states, our understanding of the atomic configurations that lead to traps remains limited. Therefore, the work presented in this thesis is aimed at investigating trap states and the QD surface on an atomistic scale. We use a combination of experimental and computational techniques to show that reduced metal sites can lead to trap-formation, and that these trap states can be dynamic in nature. In addition, we find suggestions that the QD surface is more complex than often assumed and that surface reconstructions may play a pivotal role in the delocalization of the wavefunction. Lastly, we study the formation of deep traps in CsPbBr3 perovskite nanocrystals. We find that the traditional picture of defect tolerance in these materials is incomplete and should also include the local electrostatic potential in order to explain deep traps.@en