Primary cilia are considered as sensory organelles that receive extracellular signals from the environment and transduce this information into a cellular response. Cyclic-nucleotide signaling has been proposed to be an evolutionary conserved signaling pathway, which acts not only in specialized sensory cilia but also in other cilia. 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. In particular, 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 to answer a fundamental biological question - what is the physiological function of primary cilia.