PETER MATTHIESSEN, PhD, is an aquatic ecotoxicologist who works as an independent consultant. Specializing in the study of endocrine disrupters, he has conducted extensive research into how sewage effluents cause feminization in wild male fish as well as how tributyltin-based antifoulants cause masculinization in wild female mollusks. Professor Matthiessen is a member of the UK Advisory Committee on Pesticides and Co-chair of the OECD Validation Management Group for Ecotoxicity Tests. He has contributed to the development of standardized ecotoxicity assays with sensitivity for endocrine disrupters as well as written guidance for the interpretation of the resulting data.