A comprehensive review of available longitudinal data on public opinion toward environmental issues since 1965 suggests the following conclusions regarding trends in public concern over environmental quality: (a) Environmental concern developed dramatically in the late 1960s and reached a peak with the first Earth Day in 1970; (b) such concern declined considerably in the early 1970s and then more gradually over the rest of the decade, but remained substantial; (c) the 1980s saw a significant and steady increase in both public awareness of the seriousness of environmental problems and in support for environmental protection, with the result that by the twentieth anniversary of Earth Day in 1990, public concern for environmental quality reached unprecedented levels. This supportive public opinion provides a valuable resource for the environmental movement, and the future of the movement will depend heavily on the degree to which environmentalists can effectively mobilize this support.