Phytochromes are a superfamily of bilin-based photoreceptors that mediate a wide range of light responses in plants, fungi and bacteria. In photosynthetic bacteria, they regulate gene expression of key photosynthetic components and pigment-processing enzymes. Canonical bacteriophytochromes (BphPs) are multi-domain sensor histidine kinases that undergo light-dependent auto-phosphorylation in a two-component system where the phosphoryl group is relayed to an Asp residue in a cognate response regulator, thereby triggering downstream transcriptional actions. Despite the extensive studies on bacteriophytochromes, the molecular mechanisms of light signaling and allostery remain elusive in the absence of full-length structures representing distinct signaling states. To address this challenge, we tackle a few representative BphPs using an integrated approach of biochemistry, spectroscopy, mutagenesis and structural biology. Specifically, we harness dynamic crystallography and single particle cryo-electron microscopy to provoke, probe and resolve the functional relevant structural dynamics in the truncated photosensory domains and full-length proteins. Findings are expected to elucidate the long-range signaling mechanisms in dimeric receptor kinases beyond photosynthesis and photoreceptors, which also promise to offer optogenetic solutions for biomedicine and renewable energy research. In this conference, I will present our recent cryoEM studies of BphPs that undergo large structural changes in response to light.