Heart rate variability (HRV) is a simple noninvasive measurement for investigating autonomic influence on the cardiovascular system. HRV, the beat-to-beat alterations in heart rate, may be evaluated by time domain and frequency domain methods. HRV can be used as a predictor of risk or warning sign of cardiovascular diseases. Numerous studies have shown that a reduced HRV can also be used as a predictor of hypertension, development of diabetic neuropathy, cerebrovascular disease, congestive heart failure, and lethal arrhythmic complications after an acute myocardial infarction. A few studies have also revealed that relaxation techniques can increase HRV. This article reviews the literature about HRV measurement and the relationships among HRV, hypertension, and relaxation techniques. Limitations of the review literature have also been considered to identify areas for future research.