The autonomic nervous system plays an important role in the pathology of heart failure. The single-photon emission computed tomography tracer iodine-123-metaiodobenzylguanidine ((123) I-MIBG) can be used to investigate the activity of the predominant neurotransmitter of the sympathetic nervous system, norepinephrine. Also, positron emission tomography tracers are being developed for the same purpose. With (123) I-MIBG as a starting point, this brief review introduces the modalities used for cardiac sympathetic imaging.