Publisher Summary Advances in the understanding of the formation, structure, and chemistry of the mammalian kinetochore and its associated fiber are summarized in the chapter. The initiation of chromosome movement during cell division can be correlated with the formation of a fiber, composed primarily of microtubules (MTs) and associated proteins that connect each chromosome to the polar area of the spindle. The region on the chromosome where the MTs attach is the kinetochore, and the MTs themselves are known as “kinetochore MTs” (K-MTs). The origin of K-MTs is consistent with the morphological changes that occur within the astral spindle during prometaphase, with the structure of the K-fiber, with the ultrastructural data on prometaphase in many types of cells, with the in vivo nucleation data, and with the in vivo polarity determinations. The origin of K-MTs offers an explanation for various in vivo observations that have yet to be explained by a mechanism of K-MT formation, based solely on the nucleation of MTs by the kinetochore.