<p>Photocatalysis has become a cornerstone in modern organic synthesis due to its ability to generate and manage reactive radical intermediates, thus facilitating diverse chemical processes under mild conditions. A critical yet often overlooked aspect of photocatalysis is the dynamic transformation of photocatalysts into their active forms during reactions, which fundamentally governs their reactivity, selectivity and reaction outcome. Here we term this class of catalysts ‘transformer photocatalysts’, which undergo various activation pathways such as reductive activation, acid coordination, radical substitution and deconstructive processes. By categorizing selected examples based on these activation mechanisms, we aim to highlight the typical activation modes of some common photocatalysts and elucidate the underlying principles that guide the formation and behaviour of these active species. We hope that these mechanistic insights will provide a foundation for broadening the horizon of photocatalysis and developing photocatalysts tailored to organic transformations, thus inspiring further research in photochemistry and beyond.</p><p></p>