This volume is the proceedings of a symposium held on April 6-8, 1999, in San Francisco, CA, USA, and sponsored by the US Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program (EMAP). EMAP is the primary program of the EPA's Office of Research and Development to advance the science of statistically-based ecosystem monitoring; and establish baseline conditions and trends of the nation's natural resources. The first volume in this series of EMAP-sponsored symposium proceedings addressed Monitoring Ecological Condition at Regional Scales' (1998). The current proceedings was the result of the EMAP Symposium on Western Ecological Systems' (1999), and presaged the new EMAP Western Pilot Study, which will include one third of the contiguous United States. The information found in these proceedings on the state of monitoring science, and existing monitoring programs undertaken by Federal and State agencies, academic institutions, tribal governments, and environmental protection interest groups helped to establish a starting point for embarking on the EMAP Western Pilot. This volume represents current scientific and management approaches, and the results of monitoring and assessment in the western US ecosystems. The chapter topics include regional assessments, approaches to database design, landscape considerations, water quality and land use, and focuses on watersheds, lakes and rivers, and marine coastal areas. Publication of these proceedings constitutes a sound starting point for the assessment of the ecological resources in the western US and will facilitate collaborative efforts in the development and application of sound approaches to monitoring and assessment of ecological resources in the US and abroad.