Former U.S. Senator Gaylord Nelson, Earth Day's co-founder, modeled Earth Day on anti-Vietnam War demonstrations, called "teach-ins," that were common on college campuses. "At a conference in Seattle in September 1969, I announced that in the spring of 1970 there would be a nationwide grassroots demonstration on behalf of the environment," says Nelson. "The response was electric. Telegrams, letters, and telephone inquiries poured in from all across the country." As many as 20 million Americans participated in environmental rallies, demonstrations and other activities in the 1970 Earth Day.

Since the first Earth Day, however, the environmental movement has increasingly transformed itself from a largely grassroots, citizen crusade to a professionally-organized, established special interest.