"I'll see it when I believe it" Daniel Mazia Microtubules are centrally involved in many essential cell functions, including mitosis, vesicle motility, and the control of morphogenesis. Further, they appear to be involved in the control of cell cycle progression. To carry out these tasks properly, microtubules assume a protean array of different stability states and degrees of organization and they respond rapidly to requirements of the cell by modification of their organization and stability. In the typical fibroblast cell in culture, microtubules rapidly exchange their subunits with tubulin in the cytoplasmic pool, and control of this rapid turnover appears to be essential to their intrinsic capacity to perform such tasks as the separation of chromosomes in mitosis. Microtubules are not simple equilibrium polymers, but rather, they are capable of unusual nonequilibrium dynamic behaviors. One such behavior, termed treadmilling, involving the intrinsic flow of subunits from one polymer end to the other, is created by differences in the critical subunit concentrations at the opposite microtubule ends. Treadmilling was considered by many to be an in vitro dynamic behavior that did not play an important role in microtubule function in cells. However, recent evidence has established that treadmilling is a major in vivo mechanism underlying the dynamics of microtubule arrays.