A judicious choice of photocatalyst has been shown to play a critical role for the successful realization of challenging photocatalytic transformations. In WP1 the focus is on the fundamental design and understanding of novel photocatalysts, with a focus on strongly photooxidizing and photoreducing photocatalysts based on sustainable materials. Ir(III) and Ru(II) complexes are among the most employed photocatalysts, offering a broad range of ground and excited state redox potentials coupled with long-lived exciton lifetimes. However, the main drawbacks associated with their use are their prohibitive cost, scarce availability and toxicity profiles. Five ESRs work within this work package to develop earth-abundant metal or metal-free photocatalysts with improved spectral and photophysical properties.