This discussion of bioinorganic chemistry of metal ion toxicity covered essentiality and toxicity, properties of metal ions, and the toxicity of the individual metal ions. Metal ions and other substances may be classified as essential nutrients, stimulants, innocuous, therapeutic agents, and toxic. While metal ion toxicity is usually unrelated to any essentiality, toxicity and essentiality do however share two features: a frequent interdependence among metal ions and between metal ions and nonmetals in contributing to effectiveness. The following properties of metal ions were discussed: ionic radii, stability sequences, stability ruler, metal ion hydrolysis, hard and soft acids and bases, pH dependent stabilities, preferred metal ion binding sites, and ligand exchange rates. The following metal ions were surveyed, emphasizing toxicity in humans: alkali metals, lithium (7439932), magnesium (7439954), calcium (7440702), barium (7440393), strontium (7440246), beryllium (7440417), lanthanides, aluminum (7429905), chromium (7440473), molybdenum (7439987), manganese (7439965), iron (7439896), cobalt (7440484), nickel (7440020), copper (7440508), zinc (7440666), cadmium (7440439), mercury (7439976), thallium (7440280), and lead (7439921).