In the spring of 1988 an interagency consortium of Federal Land Managers and the Environmental Protection Agency initiated a national visibility and aerosol monitoring network to track spatial and temporal trends of visibility and visibility-reducing particles. The monitoring network consists of 36 stations located mostly in the western United States. The major visibility-reducing aerosol species, sulfates, nitrates, organics, light-absorbing carbon, and wind-blown dust are monitored as well as light scattering and extinction. Sulfates and organics are responsible for most of the extinction at most locations throughout the United States, while at sites in southern California nitrates are dominant. In the eastern United States, sulfates contribute to about two thirds of the extinction. In almost all cases, extinction and the major aerosol types are highest in the summer and lowest during the winter months.