It is now widely acknowledged that frequent low-intensity fires once structured many western forests. What is not generally recognized, however, is that most of those fires were purposefully set by native people, not started by lightning. Data from the Rocky Mountains attest to the widespread use of fire by native people, as does the ecology of aspen, the only common deciduous hardwood in the West. Fire history studies all show that aspen once burned at frequent intervals, yet aspen will readily burn only when the trees are leafless and the understory dry conditions which occur only early in the spring before leaf-out and understory regrowth, or late in the fall after leaf-drop and the understory has been killed by frost. During both these periods, though, there are few lightning strikes and virtually no lightning started fires in the Rocky Mountains. Thus if aspen burned frequently in the past, as all evidence indicates it once did, then those fires must have been started by native people, who used fire to modify plant communities for human benefit. Similarly, astern deciduous forests will readily burn only when leafless, but during that time there are few lightning strikes - one of many indications that aboriginal burning was also common in the eastern U.S.