INTRODUCTION: Performing at high altitude and scuba diving impose functional limitations to the respiratory system and impair exercise performance compared to normobaria. At altitude, the partial pressure of oxygen is reduced, which decreases arterial oxygen saturation and exercise performance. Falling arterial oxygen saturation results in hyperventilation and increased pulmonary ventilation. Diving poses unique effects on the respiratory system. The work of breathing is increased from marked increased airway resistance, static lung load, and hydrostatic pressure from the water on the thoracic wall. Both altitude and diving increase the work and energy cost of breathing, resulting in respiratory muscle fatigue. Respiratory muscle training (RMT) has been employed to target the deleterious effects on the respiratory system and exercise capacity. Although the literature is sparse, RMT has been reported to decrease the work and energy cost of breathing and improve pulmonary ventilation, respiratory muscle strength, pulmonary function, and exercise capacity. This narrative review summarizes what is currently known about RMT for exercise performance at altitude and in diving, including potential mechanisms and outlines gaps in the literature.Hess H, Hostler D. Respiratory muscle training effects on performance in hypo- and hyperbaria. Aerosp Med Hum Perform. 2018; 89(11):996-1001.