In 2021, there were 488 incident cases of heat stroke and 1,864 incident cases of heat exhaustion among active component service members of the U.S. Armed Forces. The unadjusted annual rates of incident heat stroke and heat exhaustion peaked in 2018 and then declined in 2019 and 2020. Between 2020 and 2021, the rate of incident heat stroke was relatively stable (0.37 cases per 1,000 person-years [p-yrs]) while the rate of heat exhaustion increased slightly (1.40 cases per 1,000 p-yrs). In 2021, subgroup-specific rates of incident heat stroke and heat exhaustion were highest among male service members, those less than 20 years old, Marine Corps and Army members, recruit trainees, and those in combat-specific occupations. During 2017–2021, a total of 312 heat illnesses were documented among service members in the U.S. Central Command (CENTCOM) area of responsibility (AOR); 6.4% (n=20) were diagnosed as heat stroke. Commanders, small unit leaders, training cadre, and supporting medical personnel must ensure that the military members whom they supervise and support are informed about the risks, preventive countermeasures, early signs and symptoms, and first-responder actions related to heat illnesses.