Heart rate variability (HRV) is estimated using time and frequency method. This analysis allows a non-invasive assessment of both the heart and autonomic nervous system. The usefulness of HRV estimation has been confirmed in patients not only with diabetic neuropathy, tetraplegia, various heart diseases (after heart transplantation, with heart failure, after myocardial infarction, with hypertrophic and dilated cardiomyopathy, with cardiological syndrome X and with arrhythmia), but also with thyroid gland diseases and in physiological states of human being. The described information indicate a wide range and increasing usefulness of HRV analysis in medical studies.