Hydration by itself will probably not prevent altitude illness. One can easily get dehydrated at high altitude because the ambient air is so dry, and activity increases insensitive loss. Dehydration may increase the risk of developing altitude illness. Drinking enough water requires effort at altitude –melting snow or purifying a liquid source – but trip leaders report that keeping well hydrated is an important factor for success. In some parts of the world people are advised not to drink water during the day’s activities but to hydrate before and after. The timing of hydration has not been adequately studied, but I advise frequent hydration. Like any advice, it can be overdone and people have died from drinking too much water. Soup mixes, drink powders, instant eggnog, cider, cocoa, and herbal teas make water more palatable and easier to consume in quantity at altitude.