The air-water interface in natural aquatic systems is often characterized by a surface film consisting of a thin layer of surface-active organic matter incorporating inorganic and organic dissolved and particulate matter. The surface film of both marine and freshwater systems concentrates nutrients, metals, and organic matter (natural and anthropogenic) above that found in the bulk surface water. Oftentimes, the particulate matter is enriched in metal and organic content above that found for particulate matter is enriched in metal and organic content above that found for particulate matter in the bulk water, but often similar to the atmospheric aerosol. The placement of the surface film at the air-water interface indicates that its composition is a result of in-lake scavenging processes and wet plus dry atmospheric deposition. The sources for and residence times of various contaminants in the surface film are discussed.