Abstract: To monitor the quantity, direction, and duration of blue light signals, higher plants rely on a sophisticated photosensory system consisting of three blue-light receptors — cryptochrome 1, cryptochrome 2 and phototropin — and various signal transduction factors. This review focuses on advances in our understanding of the functions and signal transduction of blue-light receptors and downstream components in plants on the basis of molecular biological analysis of blue-light-responsive mutants.