The basic methodological implications of spectral analysis of beat-to-beat discrete values of systolic or diastolic arterial blood pressure and RR intervals on ECG tracings are briefly summarized. Different applications are involved in the short, medium and long-term spectral estimation, in both the parameter extraction phase and in the modelling of complex pathophysiological relationships among the cardiovascular variability signals. Autospectral, crossspectral and coherence parameters find many applicative examples for a quantitative determination of stationary conditions in the functioning of neural, mechanical, vascular, humoral mechanisms of control and others. A few examples are also reported for the detection of transients in the sympatho-vagal balance which have a considerable importance even from the clinical standpoint. That is done by means of the employment of time-varient techniques of spectral estimation.

A particular emphasis is dedicated to the comprehension of various functions involved in the physiology of the above mentioned mechanisms, through proper linear modelling which makes use of a few of the obtained spectral parameters: mechanical and neural effects may be properly quantified via a closed-loop model, whose transfer functions are estimated through least squares algorithms. An interesting and novel approach is to apply non-linear models which may take into account the well known chaotic behaviour of the beat-to-beat cardiovascular discrete series. Simple models and ways of representation are introduced and commented at this regard.