Unconventional strategies for expanding the use of solar energy have attracted significant attention in recent years. Using photon upconversion, in which low-energy photons are combined to form high-energy light, it is expected that the conventional limits in photovoltaics can be shifted upwards. This process has also been utilized in contexts of e.g., optogenetics, targeted drug-delivery, photocatalysis, and photochemistry. For solar applications the mechanism called triplet-triplet annihilation photon upconversion (TTA-UC) is of specific interest, as this process functions under low-intensity, non-coherent light. By using a donor, or sensitizer, species in conjunction with a fluorescent annihilator, triplets generated by the sensitizer from incident long-wavelength light may be converted into a highly energetic singlet state within the annihilator species in a spin-allowed TTA process. This scheme has been demonstrated for many different spectral ranges and with a variety of compounds, spanning purely organic systems, nanocrystals, metallic complexes, and metal-organic frameworks, to name a few.