For the past seven years we've been scanning the skies for planets beyond Pluto. In 2005, after a search of about half of the sky and the discovery of dozens of objects almost the size of Pluto, we finally found Eris, the first object larger than Pluto, and the first that might have been called a new planet. In addition to a new avalanche of scientific questions, this discovery drives home the need to answer a question that astronomers have been unwilling to answer for years: "What is a planet?" I'll discuss the story of the discovery and try to give a perspective on why the question of planethood is difficult and why the new class of dwarf planets was created to describe all of these objects.