It can be said that the foundation of photobiology is photochemistry. Photochemical reactions involve “bimolecular” interactions between a light quantum and a molecule, as well as the subsequent chemical and physical changes which result from this interaction. Light is always one of the initial reactants in a photochemical process. Photophysical processes are transitions which interconnect excited states with each other or excited states with the ground state, via radiative and/or radiationless processes. Photochemical processes, on the other hand, are transitions from an electronically excited state that yield structures of different configurations than the reactant ground state.