Cardiac troponin levels are routinely measured for diagnosing acute myocardial infarction. Cardiac troponin measurements also provide information concerning prognosis and the effect of early intervention in patients with acute coronary syndromes. The recent development of highly sensitive cardiac troponin assays permits detection of very low circulating levels. Use of sensitive troponin assays improves overall diagnostic accuracy in patients with suspected acute coronary syndromes, and these assays provide strong prognostic information in stable coronary artery disease and chronic heart failure. However, increased sensitivity comes with a cost of decreased specificity, and serial testing, as well as clinical context and judgment, is likely to become increasingly important in the interpretation of troponin assay results.