Plan to pack out all trash. They are not required for day hikes. Yosemite uses a trailhead quota system which limits the number of people who may begin overnight hikes from each trailhead, each day. This system is designed to avoid overcrowding and to reduce impacts to wilderness areas. Permit stations are located in Yosemite Valley at the Wilderness Center, Wawona, Big Oak Flat, Hetch Hetchy, and Tuolumne Meadows.
Permits are also available by advance reservation.
You can get a permit in advance by mail. Protect Water Quality, Wildlife and Fragile Terrain Pack out all trash and use gas stoves rather than wood fires. Use proper food storage techniques. Camp in an existing, well-used campsite at least 100 feet from water. Bring a container to carry water to your Camp at least 100 feet from lakes or streams.