Increasingly, the regulatory, remedial, and restoration actions taken to manage impaired environments are based on measurement and analysis of the public community. When an aquatic assemblage has been identified as impaired, an accurate and defensible assessment of the cause can help ensure that appropriate actions are taken. The U.S.EPA's Stressor Identification Guidance describes a methodology for identifying the most likely causes of observed impairments in aquatic systems. Stressor identification requires extensive knowledge of the mechanisms, symptoms, and stressor-response relationships for various specific stressors as well as the ability to use that knowledge in a formal method for causal analysis. This document describes a strategy for developing the Causal Analysis/Diagnosis Decision Information System (CADDIS). CADDIS is envisioned as a decision support system that will help investigators in EPA Regions, states, and tribes find, access, organize, and share information useful for causal evaluations in aquatic systems. It will include supporting case studies and analysis tools, and it will provide access to databases that contain information useful for causal evaluations. The system will be developed incrementally and iteratively, and frequent user input and feedback will be essential to the system's success.