Biomarkers - which may be enzymes, hormones, biologic substances, or other markers of cardiac stress and malfunction - appear to have growing clinical importance in heart failure. Many biomarkers appear to provide important information about the cause of heart failure or the identification of those at risk for heart failure or appear to be useful in risk stratification, in the diagnosis of heart failure, or in monitoring therapy. This issue discusses the current and potential future role of biomarkers and includes articles that explore individual biomarkers in depth.