Adaptation to altitude is a complex ability of the organism which involves primarily the cardiovascular and respiratory system in order to increase the oxygen supply for body tissues. Cardiovascular adaptations to rapid altitude challenge are regulated sympathetically and manifest themselves first of all as variations of cardiac output, systolic blood pressure and heart rate at rest. However, respiration plays an important role in compensating hypobaric hypoxia at altitude. The hypoxic ventilatory response (HVR) to alveolar hypoxia expressed as alveolar hyperventilation happens within a very short period of exposure to hypoxic conditions. This immediate adaptation response is followed by the acclimatization period, which encompasses longer lasting compensation processes such as the increase of blood hemoglobin concentration improving the arterial oxygen content. This final stage of adaptation is reached within days or weeks of persisting exposure and will vary somewhat depending on the degree of altitude. For healthy, acclimatized individuals the oxygen supply to the organism does not limit the exercise performance at moderate altitudes and higher. However, above 5000 meters (15,000 ft) long term adaptation cannot occur because oxygen-diffusion becomes a limiting factor to physical exercise capacity. Altitude-related disorders are contributing significantly to the morbidity and mortality of non-acclimatized individuals. Subjects already suffering from symptomatic underlying respiratory disease and hypoxemia at rest have a higher risk and incidence of altitude adaptation disorders even at lower altitudes. Therefore, several diseases with impaired respiratory function are contraindicated for any stay at altitude.