Polo‐like kinases (PLKs) are a group of highly conserved serine/threonine protein kinases that play a key role in processes such as cell division and checkpoint regulation of mitosis. About 80% of human tumors, of various origins, express high levels of PLK transcripts. However, PLK mRNA is mostly absent in surrounding healthy tissues. Overexpression of PLK is associated with a poor prognosis in several tumor types and a lower overall survival rate. The overexpression of PLKs in human tumors, but not in healthy nondividing cells, makes them an attractive, selective target for cancer drug development. PLK inhibitors interfere with different stages of mitosis, such as centrosome maturation, spindle formation, chromosome separation, and cytokinesis. They induce mitotic chaos and severely perturb cell cycle progression, eventually leading to cancer cell death. Several PLK inhibitors are in development and are undergoing evaluations as potential cancer treatments. This review includes an overview of PLK inhibitors in early clinical development (i.e., BI 2536, BI 6727, GSK461364, ON 019190.Na, and HMN‐214) and in advanced preclinical development (i.e., ZK‐thiazolidinone, NMS‐1, CYC‐800, DAP‐81, and LC‐445). If proof of principle is confirmed in large studies, PLK inhibitors will offer a new targeted antitumor therapy for cancer patients. This manuscript reviews the PLK inhibitors in preclinical and clinical development in the therapeutic area of oncology.