Organic luminescence refers to the phenomenon of light emission from certain organic compounds in their electronic excited states under an external stimulus. Most organic luminescent compounds are fluorescent, where emission originates from radiative decay from a singlet excited state. Fast radiative decay from these short-lived excited states can be exploited in some applications, such as optical sensors and bioimaging probes, but are detrimental for others, such as flexible scintillators and organic light-emitting diodes (OLEDs). By contrast, organic room-temperature phosphorescent (RTP) materials have much longer-lived excited states, this owing to the formally spin-forbidden intersystem crossing (ISC) and phosphorescence transitions. The population of triplet states in these materials makes them attractive for diverse applications such as photocatalysis, OLEDs and background-free bioimaging. Compared to fluorophores, organic RTP compounds are relatively scarce. This is because enabling RTP requires significant spin-orbit coupling (SOC) that is usually mediated by heavy atoms such as platinoid metals, as well as changes in orbital type between singlet and triplet excited states to conform to El Sayed's rule, which also contributes to enhancing SOC. Simultaneously, non-radiative decay needs to be sufficiently suppressed such that formally spinforbidden phosphorescence can become the major decay pathways. ISC can occur between higherlying excited states, as advanced by Wu et al. in twisted biphenyl emitters where there is mixing of donor p and acceptor p * orbitals that result in significant SOC.