The Endangered Species Act is intended to conserve at-risk species and the ecosystems upon which they depend, and it is premised on the notion that if the wildlife agencies that are charged with implementing the statute use the best available scientific information, they can successfully carry out this intention. We assess effects analysis as a tool for using best science to guide agency decisions under the Act. After introducing effects analysis, we propose a framework that facilitates identification and use of the best available information in the development of agency determinations. The framework includes three essential steps--the collection of reliable scientific information, the critical assessment and synthesis of available data and analyses derived from those data, and the analysis of the effects of actions on listed species and their habitats. We warn of likely obstacles to rigorous, structured effect analyses and describe the extent to which independent scientific review may assist in overcoming these obstacles. We conclude by describing eight essential elements that are required for a successful effects analysis.