Early observations of centrosomes, made a century ago, revealed a tiny dark structure surrounded by a radial array of cytoplasmic fibers. We now know that the fibers are microtubules and that the dark organelles are centrosomes that mediate functions far beyond the more conventional role of microtubule organization. More recent evidence demonstrates that the centrosome serves as a scaffold for anchoring an extensive number of regulatory proteins. Among these are cell-cycle regulators whose association with the centrosome is an essential step in cell-cycle control. Such studies show that the centrosome is required for several cell-cycle transitions, including G(1) to S-phase, G(2) to mitosis and metaphase to anaphase. In this review (which is part of the Chromosome Segregation and Aneuploidy series), we discuss recent data that provide the most direct links between centrosomes and cell-cycle progression.