MR spectroscopy is the only method for non-invasive detection of various aspects of cardiac metabolism in humans. While the 1H nucleus of water and fat molecules is the signal source for MR imaging, the MR spectroscopic technique allows for the study of a number of other nuclei, such as 13C, 19F, 23Na, 31P, 39K and 87Rb. Clinical applications presently are confined to the 31P nucleus. 31P-MR spectroscopy allows the non-invasive study of cardiac high-energy phosphate metabolites ATP and phosphocreatine. The phosphocreatine/ATP ratio is considered an index of the energetic state of the heart. Possible clinical indications include heart failure, valve disease and coronary artery disease. In heart failure, the phosphocreatine/ATP ratio is reduced and correlates with clinical severity, ejection fraction and prognosis. In mitral and aortic valve disease, a reduced phosphocreatine/ATP ratio may indicate the optimum timing for valve replacement. In coronary artery disease, a regional decrease of phosphocreatine during stress ("biochemical ergometry") may indicate local ischemia. Furthermore, absolute quantification of high-energy phosphates may allow diagnosis of myocardial viability. Major technical developments, leading to improved spatial and temporal resolution will be necessary to establish MR spectroscopy as a routine clinical tool.