Primary cilia are considered as sensory organelles that receive extracellular signals from the environment and transduce this information into a cellular response. Whereas in motile cilia or flagella, e.g. of Paramecium, Chlamydomonas, airway epithelial cells, or mammalian sperm, cAMP regulates ciliary beating, the role of cAMP signaling in non-motile primary cilia is much less understood. Ciliary G-protein coupled receptors (GPCRs) are believed to act as extrasynaptic signaling devices, which sense the local environment. Upon stimulation, ciliary GPCRs have been proposed to locally change the cAMP concentration in the primary cilium, thereby, controlling cellular functions like gene expression. However, the cellular responses downstream of ciliary cAMP signaling and the physiological functions are rather enigmatic. We apply an interdisciplinary approach combining optogenetics, genetically-encoded biosensors, high-resolution imaging, cell biology, systems biology, and biochemistry.