The Pluto-Charon system is the only planet-satellite system in our solar system that has not been explored by spacecraft. Therefore, the state of knowledge about this system is necessarily more primitive than about any other planet. Despite this, however, many basic facts are established. These include the radius, mass, and density of Pluto (each known to better than 10%) and the radius of Charon (known to 5%), and the mass and density of Charon (known to about 25%). Importantly, Charon is almost precisely half the size of Pluto. Because the system barycenter is known to be outside Pluto (between the two bodies), the pair constitutes a true double planet - something unique in our solar system.