In this book the authors analyse the circadian characteristics of autonomic cardiac control in norm and in pathology through the indices of heart rate variability. The starting hypothesis was that the changes in the circadian nature of the autonomic balance have both specificity and sensitivity, i.e., they have a characteristic profile in the principal cardiovascular pathologies. The material is presented in eight chapters. The first four summarise the anatomical and the functional data on the nerve control of the cardiovascular system, on the circadian nature of the different systems securing that control, and on the variability of the heart rate as a method for studying the autonomic control of the heart. The next four chapters summarise the results of our studies clarifying the profiles of the circadian characteristics of the autonomic control in healthy individuals and in patients with arterial hypertension, ischaemic heart disease and heart failure.