Development of photoinjectors is a critical research area for modern accelerators. Photoinjectors produce the high brightness electron beams that have enabled the realization among others of Ultrafast Electron Diffraction and Microscopy setups, X-ray Free Electron Lasers, Energy Recovery Linacs, high average current sources for bunched electron beam cooling and spin polarized electron beams used in colliders. All these accelerator machines rely on photocathodes to produce electron beams with precisely controlled temporal and spatial shapes, often with stringent requirements on emittance, temporal response, and spin polarization. This explores the current state of the art in accelerator photocathodes, from operational, theoretical and a materials science perspective, aiming also at identifying directions for future research and new opportunities for collaboration.