To trully allow fires to play their natural role in wilderness ecosystems, it is sometimes necessary to have large fires of long duration. Large fires are ecologically significant events that drive many other ecosystem processes. However, these fires pose significant management concerns. As a result, managers may limit the opportunity for fires to play their natural role. Risk of escape out of prescribed natural fire zones, endangerment of human life and structures, and smoke are all concerns that must be addressed before fires are allowed to grow large. Experience in California has shown that smoke is the most frequent limiting factor, while fire escape and public safety have been important issues in Wyoming and Montana. A decision to limit the size or duration of natural fires is a decision to alter natural ecosystem function. Such a decision must be made with the best information available about possible ramifications.