Cancer is one of the major cases of death in the world. Current treatment of cancer is limited to surgery, radiotherapy, and the use of cytotoxic agents, despite their well known side effects and problems associated with the development of resistance. For most forms of disseminated cancer, however, no curative therapy is available, and the discovery and development of novel active chemotherapeutic agents is largely needed. Since the development of cisplatin, an inorganic platinum complex, numerous platinum and non-platinum metal complexes were synthesized and tested for anticancer activity. Very few match the clinical efficacy of cisplatin. Ruthenium complexes were prepared to ameliorate cisplatin activity, particularly on resistant tumours, or to reduce host toxicity at active doses. Since many years a lot of scientific groups have actively worked in the field of inorganic antitumor drugs and have developed a number of Ru(II) and Ru(III) complexes, which were shown to possess good antitumor and, above all, antimetastatic properties against animal models. Ruthenium complexes are presently an object of great attention in the field of medicinal chemistry, as antitumor agents with selective antimetastatic properties and low systemic toxicity. Ruthenium compounds appear to penetrate reasonably well the tumor cells and bind effectively to DNA. In this review, the achievements in the field of medicinal chemistry, DNA binding modes, and the development status of Ru(II) and Ru(III) complexes as anticancer agents are discussed. The aim of this review is therefore that of critically examining the past and the actual work on ruthenium compounds with emphasis on their proposed role in cancer therapy.