Polo, the founding member of the family of polo-like kinases (Plks) was identified in aDrosphila screen for mutants affecting spindle pole behavior.1 Several mutants showeddefects at their spindle poles and were hence named after the magnetic poles of the earthor geo-magnetic phenomena associated with them, like Polo and Aurora. Currently, theconserved family of Plks consists of many members throughout various species. MultiplePlks are present in mammalian cells (Plk1, Plk2/Snk, Plk3/Fnk/Prk, and Plk4/Sak) andXenopus (Plx1-3), whereas in other species only one member has been identified, likePolo in Drosophila, Cdc5 in budding yeast and Plo1 in fission yeast. Plks are now viewedas important regulators of multiple functions before and during the mitotic cell division.In this review, we will focus our attention on human Plk1 and its family members Plk2-4and the many roles they play during mitosis. Furthermore, we will describe the currentlyknowledge of the regulation of these functions.