Recent demonstration that the level of sympathetic nervous drive to the failing heart in patients with severe heart failure is a major determinant of prognosis, and that mortality in heart failure is decreased by beta-adrenergic blockade with carvedilol, indicates the clinical relevance of cardiac neuroscience research. Important initial findings were observations that the plasma concentration of the sympathetic neurotransmitter, norepinephrine, is elevated in heart failure, and that overall clinical outcome is related to plasma norepinephrine concentration (although heart failure severity may be a confounder). Sympathetic nerve recording (clinical microneurography) and radiotracer methods measuring regional sympathetic activity in the heart (cardiac norepinephrine "spillover") have now largely supplanted antecubital venous norepinephrine measurements as research tools, with newer methods providing information on regional sympathetic function that was previously lacking. The cardiac sympathetic nerves are preferentially stimulated in severe heart failure, with norepinephrine release from the failing heart at rest in untreated patients increased up to 50-fold, which is similar to the level of release in healthy hearts during near maximal exercise. There is lesser stimulation of the sympathetic outflows to the kidneys and skeletal muscle. In early mild heart failure, it is only the cardiac sympathetic nerves that are activated. This preferential activation of cardiac sympathetic outflow contributes to arrhythmia development and probably to progression of heart failure and has been linked to mortality in mild and severe heart failure. The central nervous system mechanisms involved in the sympathetic nervous activation present in patients with heart failure remain uncertain. Increased intracardiac diastolic pressure seems to be one peripheral reflex stimulus with increased forebrain norepinephrine turnover being an important central mechanism.