Polo-like kinases (PLKs) are a family of serine/threonine kinases that play crucial roles in multiple stages of mitosis. PLK1 is the most studied member of the family. It is overexpressed in a wide spectrum of cancer types and is a promising target in oncology. Most of PLK1 inhibitors are ATP-competitive. Despite the structural similarities among various kinases, several inhibitors are selective. Some areas of the PLK1 active site are important for selectivity against other kinases. These include a small pocket formed by Leu 132 in the hinge region, a bulky phenylalanine and a small cysteine at the bottom and in the roof of the ATP pocket, respectively, and an unusual concentration of positively charged residues in the solvent-exposed region. Many ATP-competitive inhibitors are heterocyclic systems able to interact with the unique features of the PLK1 binding site. Other inhibitors target regions outside the ATP pocket, such as the substrate binding domain or a hydrophobic pocket, formed when the kinase is in the inactive conformation. An alternative approach to obtain specificity and to overcome drug resistance often associated with kinase inhibitors is the inhibition of the polo-box domain (PBD) of PLK1. The PBD is unique for the family of PLKs and is essential for PLK functions; so it is a useful target for the development of selective and potent inhibitors for clinical uses. In this review some PLK inhibitors are reported, focusing on chemical structures, structure-activity-relationships (SAR) and biological activities. The great potential of these compounds could open promising perspectives. Moreover, a combination of polo-like kinases inhibitors with other anticancer drugs might offer new opportunities for cancer therapy.