We use the bi-flagellate alga Chlamydomonas and the worm C. elegans to study the assembly and maintenance of motile and sensory cilia. Bi-directional movement of multi-polypeptide particles underneath the flagellar membrane is called Intraflagellar Transport (IFT). It is required for the assembly and maintenance of all sensory and motile cilia and flagella, and is hypothesized to be the mechanism by which pre-assembled axonemal 9 + 2 precursors are moved out to their tip assembly sites, and also for the movement of proteins into the ciliary membranes. Anterograde movement is powered by kinesin-II, and the retrograde movement by cytoplasmic dynein 1b. IFT particles occur in chains of varying length that are attached to the overlying ciliary membrane and the B-subfiber of the outer doublet microtubules.