Microtubules are dynamic structures that can assemble or disassemble rapidly at their ends. Their lifetimes can vary enormously, averaging less than 1 minute for cells in mitosis and about 5–10 minutes for the microtubules that make up the radial array seen in interphase animal cells. Microtubule lifetime is longer in axons and much longer in cilia and flagella. To see how these differences occur, let’s discuss the dynamic properties of microtubules and how these properties contribute to their cellular organization.