As president of the Bullitt Foundation, Denis leads an effort to mold the major cities of Pacific Northwest and British Columbia into models of sustainability for a rapidly urbanizing planet. The Foundation applies ecological principles to the design of healthy, resilient human ecosystems. Under his leadership, the Foundation designed and constructed the Bullitt Center—the world’s greenest office building—which it operates as a commercial enterprise. Denis was the principal national organizer of the first Earth Day in 1970 and took the event international in 1990. It is now the most-wide-observed secular holiday in the world.