Earth Day was first observed in Spring of 1970. An estimated 20 million people nationwide attended festivities out of which came the largest grassroots environmental movement in U.S. history, and the impetus for national legislation like the Clean Air and Clean Water Acts. By the twentieth anniversary of that event, April 22, 1990, more than 200 million people in 141 countries participated in Earth Day celebrations.

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.