Polo, the founding member of the family of polo-like kinases (Plks) was identified in a Drosophila screen for mutants affecting spindle pole behavior.(1) Several mutants showed defects at their spindle poles and were hence named after the magnetic poles of the earth or geo-magnetic phenomena associated with them, like Polo and Aurora. Currently, the conserved family of Plks consists of many members throughout various species. Multiple Plks are present in mammalian cells (Plk1, Plk2/Snk, Plk3/Fnk/Prk, and Plk4/Sak) and Xenopus (Plx1-3), whereas in other species only one member has been identified, like Polo in Drosophila, Cdc5 in budding yeast and Plo1 in fission yeast. Plks are now viewed as 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 Plk2-4 and the many roles they play during mitosis. Furthermore, we will describe the currently knowledge of the regulation of these functions.