This fact sheet describes the spatial and temporal concentration patterns of 8 persistent toxic substances that historically have been reported to threaten or potentially threaten human health and aquatic life. Some, like dieldrin and lindane, are ubiquitous throughout the Great Lakes; others, such as mirex, hexachlorobenzene and octachlorostyrene, are more localized. Spatial concentration patterns reflect historical or on-going sources. Temporal patterns are influenced by historical or on-going sources but also reflect pollution prevention activities. This fact sheet also compares ambient concentrations to water quality objectives.