This chapter presents the current perspective on the nature of photoexcited states in quantum dots (QDs). The focus is on multiple excitons and photo-induced electron–phonon dynamics in PbSe, CdSe, Si and Ge QDs, and the advocated view is rooted in the results of ab initio studies in both energy and time domains. QDs represent the borderline between chemistry and physics, exhibiting both molecular and bulk-like properties. Unlike either bulk or molecular materials, QD properties can be modified continuously by changing QD shape and size. The atomistic description of QDs complements phenomenological models, provides important details and creates new scientific paradigms. The ab initio approaches are particularly useful for studying geometric and electronic structures of QDs because then treat bulk, surface, ligands, and effects on equal footing and incorporate electronic correlation effects. Non-adiabatic molecular dynamics simulations show that the underlying atomic structure, thermal fluctuations and surface effects lift electronic state degeneracies predicted by phenomenological motels and that excitonic electron–hole interactions are strong in small QDs. These ab initio methods are used to study excited state composition, evolution and relaxation, as well as electron phonon dephasing, all with an eye towards the incorporation of QDs in solar cells.