In the last decade, our knowledge of the outer solar system has been transformed as a result of the Voyager 2 encounter with Neptune and its satellite Triton and from Earth-based observations of the Pluto-Charon system. However, the planetary system does not simply end at the distance of Pluto and Neptune. In the past few years, dozens of bodies have been discovered in near-circular, low inclination orbits near or beyond the orbit of Neptune. These bodies are now believed to be directly related to each other and to Pluto, Charon, and Triton, and as a class they define and occupy the inner boundary of a hitherto unexplored component of the solar system, the trans-neptunian region. Exploring the Trans-Neptunian Solar System reviews current understanding of the trans-neptunian solar system and makes recommendations for the future exploration of this distant realm.