High-intensity interval training (HIT) is an increasingly popular form of aerobic exercise which includes bouts of high-intensity exercise interspersed with periods of rest. The health benefits and risks as well as the optimal design of HIT are still unclear. Further, most of the research on the effects and benefits of HIT has been done in young and middle-aged adults and as such, the tolerability and effects in senior populations is less well-known. PURPOSE: To characterize HIT research that has been done in older adults including protocols, feasibility and safety as well as to identify gaps in the current knowledge. METHODS: Five databases were searched with variations of the search terms, “high-intensity interval training, high-intensity exercise” and “elderly, older adults.” These searches identified 3377 potential studies which were reviewed for inclusion. Studies were included if they were randomized controlled trials published in English in or after 2009, if the mean age of a treatment group was 65 years or older, and if the exercise protocols being tested were exclusively high-intensity or high-intensity interval training. RESULTS: Of 3377 papers identified in the initial search, 86 met the inclusion criteria. The HIT protocols used varied widely and ranged from a single session of HIT to a year-long program. These studies included both healthy and clinical populations. The greatest number of studies examined HIT in healthy subjects, followed by subjects with cardiac or vascular disease (heart failure, coronary artery disease, hypertension), metabolic dysfunction (Type-2 Diabetes Mellitus, obesity), and chronic obstructive pulmonary disease. Sample sizes ranged from 5 to 618 with 90% of studies including less than 70 subjects. The most common primary outcomes identified in the studies included changes in cardiorespiratory fitness as well as feasibility and safety of the protocols in the target populations. CONCLUSIONS: With a few exceptions, most studies had small sample sizes and many included non-clinical populations. Larger studies are needed to further evaluate the clinical effects of HIT in older adults. Additionally, there is much variety in the modes of exercise as well as the HIT protocols being used. Despite this, HIT was generally reported to be well-tolerated and safe in older populations.