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The principal function of eukaryotic cilia and flagella is usually thought to be force generation for motility of sperm cells and single-celled organisms and for the transport of fluids over epithelial cells. The importance of these functions to human reproduction and respiratory health has long been recognized. However, cilia have another important, but often unrecognized, role in sensory perception. The sensory activities are not functionally dependent on the intricate 9+2 microtubule architecture (Fig. 1a) of the motile cilium but rely on receptors in the ciliary membrane. In vertebrates, the best examples of this are found in the retina, where the outer segments of rod and cone photoreceptor cells are derived from cilia. In these cells