Contemporary exercise guidelines focus on aerobic exercise conditioning that uses exercise intensities from 40% to 85% of maximal aerobic capacity for people who are at risk for cardiovascular disease. After a position statement from the American College of Sports Medicine challenged exercise practitioners to study exercise intensity, interval training began to emerge in the literature. Interval training involves having participants exercise briefly at higher exercise intensities (90%–95% of maximum oxygen consumption) followed by a period of recovery. Although the net effect of interval training is aerobic, this style of training involves periodic excursions of exercise intensity into “anaerobic” metabolism and appears to better stimulate whole-body cardiorespiratory improvements as well as cellular signaling involved in metabolism and energy flux. Recent reports demonstrate that interval training is effective in the elderly and patients with peripheral artery disease, intermittent claudication, coronary artery disease, chronic obstructive pulmonary disease, and type 2 diabetes mellitus.