This book presents the proceedings of the 25th "Pellston Workshop," held September 16-21, 1995 at the University of Michigan Biological Station on Douglas Lake in Pellston, Michigan, where the workshop series began in 1977. Like previous workshops, participation was limited to invited experts from government, academia, and industry who were selected because of their experience with the workshop topic. The workshop provided a structured environment for the exchange of ideas and debate such that consensus positions would be derived and documented for some of the issues surrounding the science of whole effluenct toxicity testing and receiving system impact predictions. The proceedings reflect the current state-of-the-art of these topics and focus on an assessment of 1) the appropriateness of whole effluent toxicity endpoints used in routine WET methods, 2) the degree and causes of WET test method variability, 3) biotic and abiotic factors that can influence measured field responses to effluents, and 4) the relationship between effluent toxicity, ambient toxicity, and receiving system impacts.