This report is a contribution to the Department of Trade and Industry's (now Department of Energy and Climate Change) Strategic Environment Assessment SEA2 for the North Sea. It draws on a wide range of data sources to provide an overview of the chemicals used in the offshore oil and gas industry, of the chemicals already in the environment and of those released into the environment from other sources. Considering the whole sea area, it should be noted that the water samples with the highest levels of chemical contamination are found at inshore estuary and coastal sites subject to high industrial usage. Approximately 2,000 chemical products are used by the offshore oil and gas industry. In 1999 some 180,000 tonnes of chemicals were discharged into the UK sector of the North Sea. Produced water is now the main source of contaminants, having overtaken drill cuttings since oil-based muds were replaced by less harmful alternatives. 24,286 tonnes of chemicals were reported as discharged to the UKCS in produced water in 1999. As oilfields mature, the amount of produced water increases. The range of chemicals used by the offshore oil and gas industry, the means of regulating them and of monitoring their use, are discussed. Evidence of biological effects caused by the release of contaminants into the sea is reviewed.