Triplet energy transfer (EnT) photocatalysis promotes a broad range of valuable chemical transformations including cycloadditions, electrocyclic reactions, deracemizations, migrations, and rearrangements, many of which are not accessible from the ground state. Photons offer a convenient and tuneable source of energy to selectively access reactive excited state intermediates under mild reaction conditions. Perhaps the most prominent class of photochemical reactions are [2+2]cycloadditions, which construct cyclobutanes, oxetanes and azetidines. In contrast to the analogous Diels-Alder [4+2]-cycloadditions, [2+2]-cycloadditions are thermally forbidden due to incompatible ground state orbital symmetries (Figure ). The key stages of EnT photocatalysis are shown in Figure . Light energy is used to promote a triplet sensitiser from the ground state (S0) to a singlet excited state (S1), which then undergoes intersystem crossing to a triplet state (T1). Since relaxation from T1 to S0 is spin-forbidden, triplet intermediates are relatively long-lived compared with singlet excited states. Commonly employed triplet photosensitisers include conjugated aromatic ketones such as benzophenone, xanthone, and thioxanthone, which display high quantum efficiencies for population of the triplet state. The next step involves energy transfer from the triplet photosensitiser to the substrate in an overall spin-allowed process that returns the photosensitiser to the S0 level and simultaneously promotes the substrate from S0 to the reactive T1 state, which can then undergo a variety of intra-and intermolecular processes. Enantioselective versions of photochemical reactions have been developed through dual catalytic strategies that combine an achiral photosensitiser with a photochemically inactive chiral catalyst (Figure ), or by employing chiral photosensitisers (Figure ). Although powerful, these approaches do not offer a general solution, and many desirable photochemical processes are not amenable to enantioselective catalysis. Furthermore, these reactions are oxygen sensitive, often require high catalyst loadings, and are restricted to a narrow range of substrates, with even small structural changes leading to substantial reductions in activity or selectivity. In principle, enzyme active sites could offer more versatile chiral environments for mediating enantioselective photochemistry, where multiple functional elements can be accurately positioned within a single pocket. Through directed evolution these active sites could be sculpted to maximise productive interactions between the protein, substrate and photosensitiser, to deliver efficient and selective photocatalysts (Figure ). However, while a handful of natural photoenzymes and engineered biocatalysts are known to operate via photo-induced electron transfer, enzymes that operate through EnT are currently unknown. which subsequently undergoes intersystem crossing (ISC) to the triplet state (T1). EnT returns the PS to ground state, while exciting the substrate into T1. C-D. Enantioselective EnT photocatalysis can be achieved using achiral photosensitisers in combination with chiral co-catalysts or using chiral photosensitisers. 15 E. The approach to photoenzyme development described in this manuscript, involving selective installation of a genetically encoded photosensitiser into a protein scaffold and subsequent optimisation of the modified protein by directed evolution.