Radial spokes are large protein complexes that are found in most types of motile eukaryotic cilia. Although they are conserved and essential for normal ciliary beating, their exact function is not clear yet. We have reconstituted the algal radial spoke head and determined its structure using cryo-EM. We discovered that the radial spoke head is rigid and has a negatively-charged, flat surface. We are trying now to understand the functions of the radial spokes and figure out how these huge complexes are assembled.