Centrosome duplication and separation are linked inextricably to certain cell cycle events, in particular activation of cyclin-dependent kinases (CDKs). However, relatively few CDK targets driving these events have been uncovered. Here, we have performed a screen for CDK substrates and have isolated a target, CP110, which is phosphorylated by CDKs in vitro and in vivo. Human CP110 localizes to centrosomes. Its expression is strongly induced at the G1-to-S phase transition, coincident with the initiation of centrosome duplication. RNAi-mediated depletion of CP110 indicates that this protein plays an essential role in centrosome duplication. Long-term disruption of CP110 phosphorylation leads to unscheduled centrosome separation and overt polyploidy. Our data suggest that CP110 is a physiological centrosomal CDK target that promotes centrosome duplication, and its deregulation may contribute to genomic instability.