Bioinorganic chemistry sits at the interface of Chemistry and Biology and deals with metal ions in biological systems. It is a young and evolving field although its inception lies in the 19th century when metal ions bound to proteins and enzymes were first discovered. The unambiguous proof of the presence of Zn in enzymes was established in the 1930’s and elements such as Ni and Se were only known as components of important enzymes in the 1970’s. Today, compounds comprising 76 elements are either approved or under clinical trial for use in diagnostic and therapy. The human body has trace amounts of many metal ions, whose concentrations are tightly regulated through homeostasis. Transition metals are generally found bound to proteins, or in cofactors, for example in porphyrins and cobalamins, or in clusters. Na, K, Mg, Ca, Mn, Fe, Co, Cu, Zn, and Mo account for 2.5% of the human body mass. Their roles are so important that without them red blood cells could not be produced and metabolism would not function properly. It is believed that at least one-third of human proteins and one-half of enzymes require a metal ion to function. Understanding how metals bind to proteins, act as co-factors, or as inhibitors and modify processes via changes in structure and function are of utmost importance to our understanding of disease.