The cilium is an evolutionally conserved apical membrane protrusion that senses and transduces diverse signals to regulate a wide range of cellular activities. The cilium is dynamic in length, structure, and protein composition. Dysregulation of ciliary dynamics has been linked with ciliopathies and other human diseases. The cilium undergoes cell-cycle-dependent assembly and disassembly, with ciliary resorption linked with G-S transition and cell-fate choice. In the resting cell, the cilium remains sensitive to environmental cues for remodeling during tissue homeostasis and repair. Recent findings further reveal an interplay between the cilium and extracellular vesicles and identify bioactive cilium-derived vesicles, posing a previously unrecognized role of cilia for sending signals. The photoreceptor outer segment is a notable dynamic cilium. A recently discovered protein transport mechanism in photoreceptors maintains light-regulated homeostasis of ciliary length.