Cobalt is a lustrous, silvery-blue magnetic metal. Cobalt is a bioessential element due to its location at the centre of vitamin B12. Vitamin B12 plays a number of vital roles in the physiology of the human body. Cobalt is also important in treatments of radiotherapy in the form of the isotope 60Co. Other medical uses of cobalt include the detection of tumours and metastases, sterilisation of surgical equipment and the imaging of damage to the brain. Cobalt is also used in the prosthetic alloys sector, being utilised in hip, knee and dental replacements. There are inorganic cobalt complexes that elicit biological effects with potential use as pharmaceutical agents. Three classes of cobalt complexes are present: 1) complexes that directly act on biomolecules through ligand exchange, 2) complexes that modify the activity of ligated drugs and 3) complexes that are activated by bioreduction to either (I) yield a cobalt effector species or (II) release a small molecule drug. Cobalt can cause a distinctive, rapidly progressive and reversible depression of cardiac systolic function, which is readily distinguished from other causes of cardiomyopathy.