In her address to the Class of 2024, the Dean of the Yale School of the Environment (YSE) reminded the new graduates, "You will be leaders for change, and soon." It's a message every YSE student has heard since 1900, when forester Henry S. Graves and botanist James Toumey opened the first graduate school of forestry in the Western Hemisphere at Yale University. The original mission was training men to lead America's forest conservation movement. What the two professors created, though, quickly became the foremost school for educating leaders in forest and environmental sciences, policy, and practice around the world, and the model for many other schools to emulate. It remains so 125 years later. YSE faculty and graduates have been successful by adapting to ever-changing conditions in the classroom, on campus, in the field, and in the laboratory, and in state legislatures and the US Congress. They've led nonprofit conservation organizations, governmental agencies, and forest-product corporations, and served the needs of urban neighborhoods and those living in the remotest corners of the planet. The goal for faculty and students alike has been to increase our understanding of the natural world and the built landscape, and how we interact with them, while serving the public good. From the moment the school's founding director Henry Graves called the first class to order and for every succeeding generation, YSE has had a global impact unlike any other. This is its story.