Tissue magnetic susceptibility can be calculated from gradient-echo phase images using quantitative susceptibility mapping (QSM). Several clinical applications of QSM are emerging based on its sensitivity to tissue iron, myelin and deoxyhaemoglobin content. These include visualising iron in deep-brain structures in Parkinson’s disease and other dementias, evaluating microbleed burden and haemorrhages and distinguishing these from calcifications. QSM also allows quantification of venous oxygenation with functional QSM now able to detect brain activity. QSM reveals demyelination: changes in both myelin and iron content drive QSM differences in Multiple Sclerosis which may be associated with inflammation, perhaps due to iron in microglia/macrophages.