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, a problem that bedevils traditional approaches. Rather than using upstream reference sites in a river system or next-bay-over reference sites in a lake, an array of ecologically similar, least-exposed to stress sites scattered throughout a catchment or region is used. Once the reference condition has been established, any site suspected of being impacted can be assessed by comparison to the reference sites, and its status determined. The Reference Condition database, once formed, can be used repeatedly.