On July 7-9, 1996 the U.S. Geological Survey's Office of Water Quality and Toxic Substances Hydrolgy Program sponsored a workshop on mercury cycling in the environment in recognition that public concern for fish and wildlife and human health from mercury toxicity has increased substantially over the past 5 to 10 years. These concerns are manifested primarily from the issuance of fish consumption advisories in the majority of U.S. states, Canada, and several European countries due to high levels of mercury in game fish. Although the precise causes for this contamination are poorly understood, it appears to result from both source and ecosystem-specific factors. Until recently, attempts to unravel this environmental contamination problem have been frustrated by both sampling and analytical barriers. For most aquatic ecosystems, atmospheric deposition is the primary source of mercury (although there are numerous instances of geologic and anthropogenic point-source contamination cases) and the resulting aqueous concentrations of mercury are generally less than 10 nanograms per liter. The challenge to scientists is to explain the series of processes that lead to toxic or near-toxic levels of mercury in organisms near the top of the food chain (the bioaccumulation process), when aqueous concentrations and source delivery rates are so low. To adequately understand this phenomenon an interdisciplinary approach is requisite. Due to recent great strides in sampling and analytical techniques, scientists can now routinely collect representative air, water, tissue, and sediment samples, and analyze for specific mercury species. The resultant data have provided new insights into the processes controlling the transport, cycling, and fate of mercury in aquatic ecosystems. In addition, new techniques that employ isotopic tracers have provided new insights about the specific processes at the root of this contamination problem: mercury methylation and demethylation.