Eukaryotic cilia and flagella, including primary cilia and sensory cilia, are highly conserved organelles that project from the surfaces of many cells. The assembly and maintenance of these nearly ubiquitous structures are dependent on a transport system — known as 'intraflagellar transport' (IFT) — which moves non-membrane-bound particles from the cell body out to the tip of the cilium or flagellum, and then returns them to the cell body. Recent results indicate that defects in IFT might be a primary cause of some human diseases.