Bioinorganic chemistry is a leading discipline at the interface of chemistry and biology. This course attempts to unify the field of bioinorganic chemistry by identifying the principles that have emerged and arranging them in a logical and consistent order. We first consider which metal ions are used in living organisms, why nature might have chosen them, how they get into cells, and how their concentrations are regulated. We next discuss how metals bind to biopolymers, how metal binding can fold biopolymers, leading to function, and how they are inserted into their active centers. We finally treat the major roles of metal ions in biological systems.