This document discusses linear response theory and time-dependent density functional theory (TDDFT) for calculating absorption spectroscopy. It begins by motivating the use of absorption spectroscopy to study many-body effects. It then outlines how to calculate the response of a system to a perturbation within linear response theory and the Kubo formula. The document discusses using TDDFT to include electron correlation effects beyond the independent particle and time-dependent Hartree approximations. It emphasizes that TDDFT provides an exact framework for calculating neutral excitations if the correct exchange-correlation functional is used.