Polo-like kinases (Plks) are important regulators of the cell cycle. Plk1, the most studied Plk, has been implicated in regulating centrosome maturation, mitotic entry, sister chromatid cohesion, the anaphasepromoting complex/cyclosome (APC/C), and cytokinesis. Sister chromatid separation and the subsequent formation of two genetically identical daughter cells depend on the symmetrical attachment of all chromosomes to the mitotic spindle, a process called chromosome biorientation. Several mitosis-specific protein kinases have been implicated in bipolar spindle assembly and chromosome biorientation. For example, Cdk1 (cyclin-dependent kinase 1) is known to phosphorylate Eg5, a kinesin that is required for the migration of centrosomes and is thus needed for the formation of bipolar spindles. At the onset of mitosis, Plks contribute to the activation of Cdk1-cyclinB, and they are also required for the inactivation of Cdk1 and exit from mitosis. Plks are important regulators of the APC/C, a key component of the ubiquitindependent proteolytic degradation pathway.