The multiple facets of cardiac arrhythmias and their relationship with the autonomic nervous system can be investigated by studying the spontaneous heart rate behavior through ambulatory ECG recordings, an approach that complements the limitations of invasive electrophysiologic investigations. Information obtained from heart rate behavior is more reliable in the absence of structural heart disease and ventricular hypertrophy/failure, during which compensatory mechanisms involving the autonomic nervous system tend to limit reflex changes in heart rate. Thus, in such situations, less marked sinus rhythm variations preceding the arrhythmia onset do not imply a more limited influence of the autonomic nervous system, and the sensitivity of the electrophysiologic substrate may otherwise vary. These two factors may combine to form the basis of the "adrenergic paradox" that implies that the more marked the autonomic nervous system dependence of tachyarrhythmias, the less obvious its evidence. Assessment of the QT interval dynamicity may also allow one to evaluate the modulation of autonomic neural effects on the ventricular tissues. Finally, it may be difficult to distinguish clearly autonomic nervous system dependence from rate dependence: the latter frequently conditions the behavior of the trigger whereas the former mainly concerns the electrophysiologic substrate. There are many examples of the importance of the autonomic nervous system as a determinant of cardiac arrhythmias. In the atrium, either limb of the autonomic nervous system, particularly the parasympathetic limb, can generate atrial fibrillation. The absence of structural heart disease defines pure electrophysiologic substrates responsible for benign forms of ventricular tachycardia as well as potentially lethal tachyarrhythmias of the long QT syndrome and its variants. In both, the role of the autonomic nervous system is essential, although the therapeutic consequences are crucial only in the latter. In the presence of heart disease and, in particular, heart failure, the autonomic nervous system behavior is more difficult to assess than in the absence of structural heart disease. This does not mean that its role is less crucial. In this situation the beneficial effects of beta blockers may be as important as in normal hearts although physicians should be more cautious when heart failure is present.