In this chapter we present an introductory overview of the basic theoretical concepts of computational molecular photophysics. First, the nature and properties of electronic excitations are considered, with special attention to transition moments and vibrational contributions. Then, the main photophysical processes involving the electronic excited states are examined, focusing in particular on nonradiative deactivation phenomena. Finally, we present a brief review of computational methods commonly applied for the description of molecular excitations. Special emphasis is given to the configuration-interaction (CI) method and the timedependent density functional theory (TD-DFT), discussing some technical details and outlining advantages and limitations.