The chemical and biological effects of thiosemicarbazones have received considerable interest from medicinal chemists for many years. This is attributed to, first, their wide pharmacological utility that includes antineoplastic, antibacterial, antiviral, and antifungal activity, and second, their versatility as ligands that allows them to give rise to a great variety of coordination modes. The antineoplastic activity of thiosemicarbazones has been continually examined since the 1950s, where some compounds of this general class were found to have antileukemic activity. The pronounced antineoplastic efficacy of these ligands has been widely attributed to their inhibition of the mammalian enzyme ribonucleotide reductase (RR). The ability to inhibit RR is of particular importance owing to the role of RR in the de novo synthesis of deoxyribonucleotides required for DNA replication and repair. Considering this, these compounds have often been described as RR inhibitors without mention of the property that actually leads to this effect, namely, metal chelation. Apart from the inhibition of RR, it is also known that thiosemicarbazones are typically excellent chelators of transition metals such as iron (Fe), copper (Cu), and zinc (Zn). Such ability for metal chelation is also an attractive strategy in developing anticancer drugs because of the high requirement of neoplastic cells for essential metals needed in growth and proliferation. In fact, thiosemicarbazones were the first class of chelators to be comprehensively assessed as antineoplastic agents against cancer cells in vitro and in clinical trials. Considering the authors' recent contributions to this field and their new findings regarding the mechanisms of action of these agents, including that they form cytotoxic redox active metal complexes, this Perspective will focus on understanding the antineoplastic activity of thiosemicarbazones from the point of view of chelation. To do this effectively, introduction to the roles of metals in cellular proliferation is required in addition to an evaluation of the development of chelators for the treatment of cancer and other diseases.