This assessment of seismic monitoring in the US, by the US Geological Survey, evaluates the existing systems for seismic monitoring and also discusses the advantages and importance of a modernized monitoring system. Furthermore, recommendations are made for an Advanced National Seismic System. In order to incorporate multiple perspectives on seismic monitoring needs and issues, the report was created with the involvement of a "broad cross section of the seismic monitoring community." Fifty participants attended a three-day workshop in Denver, CO in June, 1998 to develop this report.