This chapter presents a series of more specific luminescence processes. It discusses the physics required to understand excitons and optical processes in molecular materials. The molecular exciton is fundamental to organic light emitting diode (OLED) operation. It is possible for the molecule to be excited to form an exciton by electrical means rather than by the absorption of a photon which is the situation in OLEDs. The most important applications of LEDs and OLEDs are for visible illumination and displays. This requires the use of units to measure the brightness and color of light output. To determine the probability of occupancy of states in the bands, people use Fermi‐Dirac statistics. The chapter introduces photometric units and relates them to radiometric units. Finally, it summarizes the key concepts underlying p‐n junction operation for reference and detailed quantitative treatments.