This chapter examines photophysics. Several of the pathways for loss of electronic excitation do not lead to any chemical change. Instead, the energy is used in a physical process such as the emission of light as luminescence, or it becomes lost by quenching. Alternatively, the excitation energy can populate a new level in the same or a different molecule by intramolecular or intermolecular energy transfer; the new state of the new molecule excited by the energy transfer process can, of course, emit its own radiation. Luminescence, quenching, and the two types of energy transfer are important aspects of photophysics. Luminescent emission provides some of the most reliable information about the nature of primary photochemical processes. The chapter also looks at fluorescence, phosphorescence, and chemiluminescence.