The aim of this paper is to review how preexisting pulmonary diseases can be affected by altitude exposure. Obstructive (asthma and chronic obstructive pulmonary disease or COPD) and restrictive (interstitial pulmonary fibrosis), as well as pulmonary vascular diseases, will be considered, and the goal will be to provide insight and tools to clinicians to optimize the medical condition and thus the life-style of these patients. The underlying pathophysiologies and the effect of hypobaric hypoxia on these diseases will be reviewed such that techniques to assess patients will be appropriate. Therapeutic interventions, including the use of supplemental oxygen, in light of the underlying pathologic processes, will also be discussed.