The determination of the extent of the non-viable tissue after myocardial infarction has a major impact on further treatment of patients. During acute myocardial infarction, total sodium content of the tissue is elevated. This is caused by discontinuation of ion homeostasis, edema formation and membrane rupture. The situation is a different one in the chronic phase of scar formation, where cell migration causes changes in the ratio of extra- and intracellular volume. The accumulation of sodium causes an increase in the signal in 23Na magnetic resonance imaging. The differences in intra- and extracellular sodium concentration modulate total sodium content and can be used as a natural, intrinsic contrast. In animal experiments, 23Na magnetic resonance imaging allows the non-invasive determination of infarct size. As neither stunned nor hibernating tissue shows elevated total sodium content of the tissue, the method is able to discriminate viable and non-viable tissue. A small number of initial clinical studies show promising results for the use of this technique in humans. The development of 23Na magnetic resonance imaging and the current status of the application are described.