The cilium is a complex organelle, the assembly of which requires the coordination of motor-driven intraflagellar transport (IFT), membrane trafficking and selective import of cilium-specific proteins through a barrier at the ciliary transition zone. Recent findings provide insights into how cilia assemble and disassemble in synchrony with the cell cycle and how the balance of ciliary assembly and disassembly determines the steady-state ciliary length, with the inherent length-dependence of IFT rendering the ciliary assembly rate a decreasing function of length. As cilia are important in sensing and processing developmental signals and directing the flow of fluids such as mucus, defects in ciliogenesis and length control are likely to underlie a range of cilium-related human diseases.