Most people think of a skeleton as a solid and static framework upon which complex structures are elaborated. From what we have learned in the past decade about the cytoskeleton, it seems certain that the 'skeleton' part of the term is a bit misleading. It is clear now that the polymers that constitute the cytoskeleton, actin filaments, microtubules, and intermediate filaments, are all in fact ever-changing dynamic infrastructures of cells. Recently, advances have been made in the study of the cellular dynamics of one of the prominent components of the cytoskeleton, the microtubules. Observations in the past year have revealed some fundamental in vivo behaviors of these polymers, during interphase, during mitosis, and during the elaboration of postmitotic axonal microtubule arrays. These observations are important for the understanding of cytoplasmic organization in many types of cells.