The organization and function of microtubules change dramatically during the cell cycle. At the onset of mitosis, a radial array of microtubules is broken down and reorganized into a bipolar spindle. This event requires changes in the dynamic behavior of individual microtubules. Through the use of Xenopus laevis egg extracts, a number of proteins affecting microtubule behavior have been identified. Recently, progress has also been made towards understanding how the activities of such microtubule-affecting proteins are regulated in a cell cycle-dependent manner. It is hoped that understanding how microtubule behavior is controlled during the cell cycle in vitro may illuminate the role of microtubule dynamics in various cellular processes.