Buried wastes are isolated from the environment by barriers constructed entirely or in part of compacted soil. The chief concern in barrier design has been to isolate the waste in the short term by preventing movement of water into and through the waste. However, in the long term a variety of mechanisms can act to compromise the isolation. The mechanisms of long term failure include initial flaws in barrier construction, shrink-swell cycles, freeze-thaw cycles, erosion, subsidence, root intrusion, and animal intrusion. Evidence for action of all of these mechanisms is summarized. The likelihood of long-term failure suggest that either perpetual care must be provided for buried hazardous wastes, or the waste sites must be designed to withstand long-term threats to barrier integrity.