The natural soundscape of the Greater Yellowstone Ecosystem (GYE) has many biological sounds with important ecological functions for reproduction and survival. Birds, mammals, amphibians, and insects often need to hear or produce sounds to attract mates, detect predators, find prey, and/or defend territories. The natural soundscape of the GYE delights visitors during the fall elk rut, during birds’ spring choruses, along rushing streams, and in the delightfully still and profoundly quiet days and nights of winter. Natural soundscapes are a resource and are protected by National Park Service policies. Many park visitors come to national parks to enjoy serenity and solitude and expect to hear sounds of nature. Sounds associated with human activity, including road traffic, aircraft, and snowmobiles often impact these natural soundscapes and are an important and growing source of concern. Yellowstone and Grand Teton National Parks initiated a soundscape monitoring program in 2003.