Internal frames have much to offer backcountry skiers, winter campers, and high angle climbers. They allow what one calls a "live load", riding closer to the back and therefore the wearer's center of gravity. However, the backpacking industry (including most retail outlets) have unfairly designated external frame backpacks as obsolete artifacts from the Jurassic Age of outdoor equipment. In Al's opinion -- drawing from his and other folks' experience -- an external frame is more efficient for carrying a heavy load over long relatively flat distances; and, in almost all cases, an external frame pack is significantly less expensive, lighter, easier to load properly and is also far better ventilated for summer use. So don't throw your old Kelty D4 in the trash (unless you want to purchase a new Kelty external from Moor & Mountain!