Background Cardiac rehabilitation ( CR ) for patients with cardiovascular disease has traditionally involved low- to moderate-intensity continuous aerobic exercise training ( MICT ). There is growing and robust evidence that high-intensity interval training ( HIIT ) shows similar or greater efficacy compared with MICT across a range of cardiovascular and metabolic measures, in both healthy populations and populations with a chronic illness. However, there is understandable concern about the safety aspects of applying HIIT in CR settings. This systematic review analyzed safety data drawn from recent proof-of-concept studies of HIIT during CR among patients with cardiovascular disease. Methods and Results We included trials comparing HIIT with either MICT or usual care in patients with coronary artery disease or heart failure participating in tertiary care services, such as phase 2 (outpatient) CR . Adverse events occurring during or up to 4 hours after an exercise training session were collated. There were 23 studies included, which analyzed 1117 participants ( HIIT =547; MICT =570). One major cardiovascular adverse event occurred in relation to an HIIT session, equating to 1 major cardiovascular event per 17 083 training sessions (11 333 training hours). One minor cardiovascular adverse events and 3 noncardiovascular adverse events (primarily musculoskeletal complaints) were also reported for HIIT . Two noncardiovascular events were reported in relation to MICT . Conclusions HIIT has shown a relatively low rate of major adverse cardiovascular events for patients with coronary artery disease or heart failure when applied within CR settings.