Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes (TADF-OLEDs) comprehensively introduces the history of TADF, along with a review of fundamental concepts. Then, TADF emitters with different colors, such as blue, green, red and NIR as well as white OLEDs are discussed in detail. Other sections cover exciplex-type TADF materials, emerging application of TADF emitters as a host in OLEDs, and applications of TADF materials in organic lasers and biosensing.