This chapter reviews the work on tektins from cilia and flagella and presents further evidence for their similarity to intermediate filament proteins and for the possibility that this class of proteins occurs in other microtubule systems, particularly centrioles and spindles of dividing cells. Some of the possible functions of tektins are also considered. Cilia and flagella constitute an important class of microtubules, and much can be learned from them that may be applicable to general microtubule structure and function. These organelles may be motile, such as the sperm flagella and epithelial cilia of the respiratory and reproductive tracts, or they may be nonmotile, such as primary cilia and the sensory cilia of auditory hair cells, olfactory cells, and retinal rods and cones. The formation of primary cilia and replication of their parent centrioles are specifically timed events in the cell cycle. A number of genetic mutations and pathological conditions are known to affect the structure and function of these organelles. For biologists, cilia and flagella from protists and marine invertebrates have become useful systems for the study of microtubules.