Publisher Summary This chapter discusses the structure and composition of the mitotic apparatus in relation to Ca regulation and describes the existence, timing, and location of endogenous Ca transients. The evidence for the role of Ca gathered from the experimental modulation of the extracellular and intracellular environments is also presented in the chapter. Microtubules (MTs) are a major component of the mitotic apparatus; they attach to the chromosomes and participate in the motile events that ensure their equipartition to daughter cells. Spindle MTs are considered Ca-sensitive structures. In addition, the mitotic apparatus contains an extensive array of Ca-related factors—that is, there are elements that are Ca regulated, there are proteins that can respond to Ca signals, and there are Ca stores and associated pumps to control an ion. Absolutely crucial to a possible connection between Ca and mitosis is the presence of factors—notably proteins—that are able to recognize the ion and respond to changes in its concentration. Calmodulin emerges as the most prominent candidate because it potentiates the depolymerization of MTs by Ca and is known to be localized in the mitotic apparatus.