Both the Hartree-Fock method and Kohn-Sham DFT provide a framework to calculate the ground state of an interacting many-electron system. There are various ways to obtain information about excited states in these frameworks, e.g., by time-dependent formulations (explicit or within linear response). Alternatively, one can consider electronic excitations as perturbations to the ground state and obtain again effective single-or two-particle formulations for the excitations and their energies within the framework of perturbation theory with many-body Green's functions.