Much of our knowledge about microtubules has come from detailed morphological, biochemical, and cell biological studies. As more is learned about these organelles, questions regarding the in vivo regulation of their expression and function become increasingly important. Genetics provides an approach to address these more subtle questions in the living cell. Mammalian mutants with microtubule alterations have been isolated using selections for resistance to the cytotoxic effects of a number of antimitotic drugs. A subset of these mutants have clearly defined alterations in alpha- or in beta-tubulin, and these have been used to explore the mechanisms by which mammalian cells acquire resistance to this class of drugs. In addition, the mutants are providing valuable insights into how tubulin expression is regulated, into what factors determine the extent of microtubule assembly in living cells, into the domains of tubulin that are involved in assembly, and into the role of microtubules in essential cellular processes.