B-type natriuretic peptide (BNP) is a cardiac neurohormone released as pre-proBNP and then enzymatically cleaved to the N-terminal-proBNP (NT-proBNP) and BNP upon ventricular myocyte stretch. Blood measurements of BNP and NT-proBNP have been used to identify patients with heart failure (HF). Clinical considerations for these tests include their half-lives in plasma, dependence on renal function for clearance, interpretation of their units of measure, and the rapid availability of the test results. The BNP assay is currently used as a diagnostic and prognostic aid in HF and as a prognostic marker in acute coronary syndromes (ACS). In general, a BNP level less than 100 pg/mL excludes acutely decompensated HF. In the absence of renal dysfunction, NT-proBNP has also been shown to be of diagnostic value in HF, related to HF severity, predictive of sudden death, and prognostic for death in ACS. This article will sort out the literature concerning the use of these peptides in a variety of clinical scenarios.