SUMMARY It has been 20 years since the phenomenon of centralpair rotation in eukaryotic flagella was reported(Omoto, and Kung, 1979). At that time, a model wasproposed in which the central pair functions as adistributor to regulate dynein activity among the outerdoublet microtubules. The video evidence obtainedsince then and gathered together in this essay is con-sistent with this model. Geometric arguments indicatethat there must be circumferential and longitudinalregulation of shear forces to produce effective bendingmotion of an axoneme. Central pair microtubules areideally situated to perform this regulatory function.The regulation of outer doublet sliding by the centralpair, together with the rotation of the latter, where thisoccurs, may explain the wide diversity of two- andthree-dimensional flagellar and ciliary waveforms thatis found in organisms using the same basic 9 1 2structure. ACKNOWLEDGMENTS We thank Mike McLaughlin of Material Media Services, Washing-ton State University for converting 16-mm film to video format, andDenise A. Palmen (Technical Services, Washington State University)for editing the videos, assembling the QuickTime movies, and pro-ducing the jpeg images to use as figures. The research based onvideo images shown in this essay was first published by Kamiya(1982), Omoto and Witman (1981), Gibbons