Genetics has become an important tool for studying microtubule structure and function. Mutations in genes that encode microtubule proteins have been isolated in several, evolutionarily diverse organisms. These mutations have been, and will increasingly be, of great value in determining which cellular events are microtubule mediated, in determining which genes encode the microtubule proteins involved in a particular cellular event, and in determining the mechanisms of resistance to anti-microtubule drugs. These mutants also have great potential, which is just beginning to be realized, for identifying proteins other than α- and β-tubulin that are essential to microtubule function and for determining the mechanisms of microtubule-based force production in mitosis and organellar movement.