Severe illnesses like heart disease and cancer are accompanied by pathological changes in biological rhythms. Applying the cutting edge advances in research of biological rhythms, such as Heat Rate Variability, allows the developing of diagnostic therapeutic approaches towards effective health promotion, as well as prevention and treatment of illness. These techniques applied by health professionals and psychologist allow to develop a window in to the functions of the autonomic nervous system. The application of this research and practice is wide and far reaching.