The content of a Special Workshop on Heart Rate Variability Biofeedback (HRV-BF), held during the 41st Annual Meeting of the Japanese Society of Biofeedback Research in Kamakura is described. HRV-BF is accompanied by slowed respiration at a rate of 0.1Hz, which is thought to induce high-amplitude oscillations of the heart rate, blood pressure, and vascular tone. Respiration at the rate of 0.1Hz causes the HF and LF components in HRV to synchronize and merge at a frequency in which resonance occurs between processes mediated by respiratory sinus arrhythmia and those related to baroreflex activity and produces large increases in the magnitude of HRV. This phenomenon is called the resonance effect. In this workshop, in order to introduce some key points of HRV-BF, we presented the following, (1) a demonstration of HRV, (2) mechanisms of HRV-BF: resonance in the cardiovascular system, (3) the effects of HRV-BF, and (4) how to determine the resonance frequency. Although a number of HRV-BF studies have demonstrated that the technique has clinical utility for the treatments of physical and mental disorders involving autonomic nervous system dysregulation, further investigations of its basic mechanisms and the development of a theory of HRV-BF are warranted.