BLUF domains are flavin-binding photoreceptors that can be reversibly switched from a dark-adapted state to a light-adapted state. Proton-coupled electron transfer (PCET) from a conserved tyrosine to the flavin that results in a neutral flavin semiquinone/tyrosyl radical pair constitutes the photoactivation mechanism of BLUF domains. Whereas in the dark-adapted state PCET occurs in a sequential fashion where electron transfer precedes proton transfer, in the light-adapted state the same radical pair is formed by a concerted mechanism. We propose that the altered nature of the PCET process results from a hydrogen bond switch between the flavin and its surrounding amino acids that preconfigures the system for proton transfer. Hence, BLUF domains represent an attractive biological model system to investigate and understand PCET in great detail. © 2011 American Chemical Society.