This episode covers AMS, the most common altitude illness caused by insufficient acclimatization to hypoxia above 2,500 meters. Symptoms include headache, nausea, dizziness, fatigue, and insomnia. We examine the role of mild cerebral edema, how patent foramen ovale (PFO) increases risk, and how prevention focuses on gradual ascent, hydration, and medications like acetazolamide. Treatment includes rest, oxygen, and descent if symptoms worsen.