Following the serendipitous discovery of the anticancer activity of cisplatin over 50 years ago, a deep understanding of the chemical and biochemical transformations giving rise to its medicinal properties has developed allowing for improved treatment regimens and rational design of second and third generation drugs. This chapter begins with a brief historical review detailing initial results that led to the worldwide clinical approval of cisplatin and development of the field of metal anticancer agents. Later sections summarize our understanding of key mechanistic features including drug uptake, formation of covalent adducts with DNA, recognition and repair of Pt-DNA adducts, and the DNA damage response, with respect to cisplatin and oxaliplatin. The final section highlights known shortcomings of classical platinum anticancer agents, including problems with toxicity and mutagenicity, and the development of resistance and enrichment of cancer stem cells brought about through treatment. Instances where specific differences in the response or mechanism of action of cisplatin versus oxaliplatin have been demonstrated are discussed in the text. In this manner the chapter provides a broad overview of our current understanding of the mechanism of action of platinum anticancer agents, providing a framework for improving the rational design of better Pt-based anticancer agents.