Rehydrate by soaking or simmering. Some, such as milk, reconstitute easily in cold water. Others, such as dried beans, require long cooking to rehydrate and soften. Because of their durability and ease of preparation, these dried foods will most likely form a major portion of most back-packers diets. Cost tends to be low to moderate, but can get rather high depending on how exotic, or where you buy from.
Some good sources are backpack suppliers, health food stores, and ethnic grocers.
They require only the pouch to be dropped into boiling water for 5 to 10 minutes before serving, thus saving a messy pot. For rice and pasta, a perforated pouch allows cooking in 5 to 10 minutes in the same pot of boiling water as another dish. Retorts are heavier and more expensive than many other freeze dried or low moisture foods, but they're very palatable and come in a variety of complete entrees. They're non-perishable if the pouch remains intact. If the pouch becomes punctured, spoilage can occur because preservatives are rarely included.