Based on data from the U.S. Fire Administration's (USFA's) National Fire Incident Reporting System (NFIRS) and the National Fire Protection Association's (NFPA's) annual fire department experience survey, NFPA estimates that during 2007-2011, local fire departments responded to an average of 334,200 brush, grass or forest fires per year. In most, less than one acre burned. These incidents accounted for 24% of all fires reported to local fire departments. This study examines the circumstances and causal factors of: a) brush or brush and grass mixture fires; b) grass fires; c) forest, woods, or wildland fires; and d) total brush, grass, and forest fires, including unclassified natural vegetation fires. One in five was intentionally set. The most common heat source was a hot ember or ashes. Open burning, high winds, and smoking materials were also frequent factors. Lightning accounted for a larger percentage of forest, woods, or wildland fires than the other types of natural vegetation fires.