Microtubules are the ubiquitous components of eukaryotic cells and are organized around specific cellular structures referred as “microtubule-organizing centers” (MTOCs). This chapter describes the functional components of MTOCs. Three types of MTOCs are found in all cell types: (1) centrosome, or equivalent structure, which organizes the cytoplasmic microtubule array of interphase cells, (2) spindle poles, which organize the microtubules of the spindle apparatus, and (3) kinetochores, which are specialized regions at which chromosomes attach to the microtubules of the spindle apparatus. MTOCs of plants are usually associated with membranous structures, such as the nuclear envelope, the plasma membrane, and membrane-bound vesicles. During interphase, a single centrosome serves as the focal point for most of the microtubules of the cytoskeleton. A centrosome is composed of a cloud of electron-dense material, which, in animal cells, is usually associated with a pair of centrioles. The fundamental properties of MTOCs include the nucleation, orientation, and anchoring of microtubules. In addition, MTOCs affect not only the frequency with which microtubules nucleate but also determine the structure of the assembled microtubules.