The Great Lakes Herring Gull Monitoring Program has annually provided information concerning levels of environmental contaminants in herring gull eggs since 1974, making it one of the longest running biomonitoring programs in the world. The program was initiated in response to observations of poor reproductive success in colonial waterbirds on the Great Lakes.Initial studies examined the role of halogenated hydrocarbons (HAHs) in causing this reproductive dysfunction. By the late 1970s, reproductive success in herring gulls had improved greatly and emphasis was placed on developing more sensitive indicators to measure the subtle effects associated with HAH exposure. Geographic and temporal trends in Great Lakes contamination were also elucidated. Analysis of herring gull tissues led to the identification of HAHs (mirex, photomirex, polynuclear aromatic hydrocarbons, chlorobenzenes, dioxins) previously undetected in Great Lakes upper trophic level biota. Data collected as part of this program have improved our understanding of contaminant sources and fate in the Great Lakes and have provided us with a means to assess our progress in controlling contaminant inputs. The extensive nature of this dataset has allowed detailed examination of the factors that regulate contaminant levels in this species. Most monitoring programs rely on less extensive datasets for the interpretation of environmental trends and may benefit from the mechanisms identified here. Research has also identified other stressors, e.g., dietary deficiencies, that may affect the success ofGreat Lakes herring gull populations. Ongoing monitoring of this species will continue to provide new insights into the dynamic Great Lakes ecosystem.Key words: herring gull,Larus argentatus, Great Lakes, environmental monitoring,halogenated hydrocarbons, contaminants.