Our task is to develop theoretical concepts, implement them into computational methods and interpret experimental results; or provide genuine predictions. Our theoretical description is based on ab-initio methods, where we describe the nanostructures atom by atom. This atomistic description allows us to accurately describe colloidal semiconductor clusters, made of a few hundred atoms, all the way to epitaxial quantum dots made of millions of atoms. Our calculations of correlated many-body wave functions lead to accurate optical properties. These are relevant in the modern fields of quantum information science and solid-state quantum optics.