High-altitude illness is a continuum of syndromes that includes the potential for death. Understanding how to prevent altitude illness and how to treat it if it occurs is fundamental to trip planning and is an important role of medical advisors to travelers. Gradual ascent, along with pharmacotherapy where indicated, are the mainstays of prevention. Travelers with certain chronic medical conditions may require additional pretravel counseling and preparation. Diagnostic recognition of the differing manifestations of high-altitude illness as well as recommendations on appropriate therapeutic options are discussed in detail.