Organic light-emitting diodes (OLEDs) have now matured as a technology and are now integrated in efficient, flexible and ultra-high contrast next-generation displays. Organic thermally activated delayed fluorescence (TADF) materials are increasingly viewed as an attractive alternative emitter class to phosphorescent complexes that contain noble metals as they do not contain scarce elements yet can likewise harvest both singlet and triplet excitons in the device to generate light at comparable efficiencies. Unlike phosphorescent OLEDs that funnel the emitting excitons via the lowest-lying triplet excited state, TADF compounds convert triplet excitons into singlets via reverse intersystem crossing (RISC), which is possible at ambient temperatures due to the small S1-T1 energy gap, DEST. Compounds that possess a small DEST show a small overlap of the electron density between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). To adhere to this requirement, the great majority of organic TADF emitters are based on a highly twisted donor-acceptor architecture and emit from a long-range charge transfer (LRCT) state. This strategy, however, leads to molecules where there is a large degree of structural relaxation in the excited state that manifests in broadband emission with full width half maximum (FWHM) values of around 70-100 nm. This is unattractive to the display industry as the color purity of these devices is poor and color filters must therefore be used to meet the industry standards for red, green and blue pixels. Hence, significant effort has been devoted in recent years to suppress vibrational and conformational relaxation within the emitter through the development of narrowband emitters. An exciting molecular design strategy to achieve these requirements was introduced by Hatakeyama and co-workers and relies typically on p-and n-doped nanographenes. Termed multiple resonance (or multiresonant) TADF (MR-TADF) emitters, these compounds possess small-to-moderate DEST values, narrowband emission (with FWHM typically < 30 nm) and limited positive solvatochromism due to the short-range charge transfer (SRCT) nature of the emissive S1. Maximum external quantum efficiencies (EQEmax) as high as 34% for deep blue, 37% for sky-blue, 37% for green and 36% for red have been achieved for OLEDs containing MR-TADF emitters comprised of boron-nitrogen (B/N) doped polycyclic aromatic frameworks. Recently, the library of MR-TADF emitters has been expanded to contain boronoxygen (B/O)-based emitters and carbonyl-nitrogen (C=O/N)-based emitters as well as C=O/N/O-, C=O/N/S- and C=O/N/SO2-based 28 emitters.