The twentieth century, with the generalized use of electrocardiography (ECG or EKG) in clinical settings, marked the start of the quest to unravel the relationship between heart rate variability (HRV) and disease. The details of the physiological basis of HRV remain an open question and are the subject of intensive research. A number of parameters have been defined to analyze HRV and they can be organized into two main categories: time-domain and frequency-domain parameters. The variability of the heart rate can also be expressed in terms of frequency such that rapid (or slow) changes in heart rate are described by high (or low) frequencies. The study of HRV requires equipment that is capable of recording the heartbeat continuously for some periods of time (for example, detection of very low frequency in HRV requires recordings of at least 24 h) and without causing distress to the subject under study.