But it isn't always easy. The National Park Service operates some campgrounds, but management of hotels is contracted out to private concessionaires. These businesses have varying policies regarding reservations and cancellations. Many accept reservations one to two years in advance and require cancellations at least two or three days before arrival to avoid penalties. Many visitors make reservations far in advance and end up changing their plans, creating vacancies where there weren't any.
Some lodgings consistently sell out faster than others.
Rooms may be available, just not in the place you wanted. If you're in the park, inquire in person. The best time is late afternoon after registration personnel have calculated no-shows and cancellations. If all else fails, most parks have communities just outside their boundaries with a healthy stock of rooms. Even though you didn't get a room in the park, at least you have an alternative to sleeping in your car.