Present in organisms ranging from yeast to man, homologues of the Drosophila Polo kinase control multiple stages of cell division. At the onset of mitosis, Polo-like kinases (Plks) function in centrosome maturation and bipolar spindle formation, and they contribute to the activation of cyclin-dependent kinase (Cdk)1-cyclin B. Subsequently, they are required for the inactivation of Cdk1 and exit from mitosis. In the absence of Plk function, mitotic cyclins fail to be destroyed, indicating that Plks are important regulators of the anaphase-promoting complex/cyclosome (APC/C), a key component of the ubiquitin-dependent proteolytic degradation pathway. Finally, recent evidence implicates Plks in the temporal and spatial coordination of cytokinesis.