New display technologies such as organic light-emitting diodes (OLEDs) have become a key industry of information technology. Although great breakthrough has occured in OLED technologies such as materials and stack design processes, currently the research and development still mainly rely on trial-and-error approaches, and the understanding of the physical mechanism of the device remains mostly qualitative and empirical. In this article, the physical theories of OLEDs are systematically introduced and reviewed, especially focusing on how to physically describe the amorphous disordered nature of molecular systems, how to describe charge transport and excitonic processes, how to calculate the optoelectronic properties of devices, and how to apply these theories to experimental OLED research and development.