Larceny and petty theft are the most common crimes in national parks. They are often crimes of opportunity. For example, about 50 percent of the motor vehicles stolen in the Rocky Mountain region in recent years have had keys left in the ignition. Many people let their guard down when they are visiting the parks. Vandalism is another problem, involving defacing signs and damaging trails, barricades, and fences. Sometimes vandalism is done for revenge when park rangers enforce park regulations. Fires are set to inconvenience the rangers or to distract them from a poaching operation. To fight crime in the parks, many rangers are commissioned law enforcement agents, authorized to carry weapons, arrest people, and perform other law enforcement mandates. Those rangers receive 400 hours of law enforcement training initially and 40 hours of refresher training every year afterwards. For example, Yosemite, one of the largest parks, has 180 rangers during the busy season, 110 of whom have law enforcement commissions. Each park has a protection plan tailored to its own situation, but with a base of required actions, depending on the park's size, layout, location, and number of visitors. There is a direct correlation between the number of visitors to a park and the number of crimes that occur there. However, beyond larceny and vandalism, the national parks have no serious crime problems. Comments from rangers at four Park Service properties and photographs are included.