Aquatic ecosystem assessment is a rapidly developing field, and one of the newer approaches to assessing the condition of rivers and lakes is the Reference Condition Approach. This is a significant advancement in biomonitoring because it solves the problem of trying to locate nearby control or reference sites when studying an ecosystem that may be degraded. This book describes the basic methods involved in selecting and sampling appropriate reference sites, comparing test sites to appropriate reference sites using predictive modeling, and determining whether or not test sites are in the reference condition. This provides a rapid assessment method that can deal with everything from large-scale, national issues to local-scale problems with the same approach, and often parts of the same database.