This paper explores the opportunities and constraints to project-based environmental assessment as a means to support the assessment and management of cumulative environmental effects. A case study of the hydroelectric sector is used to determine whether sufficient information is available over time through project-by-project assessments to support an adequate understanding of cumulative change. Results show inconsistency from one project to the next in terms of the components and indicators assessed, limited transfer of baseline information between project assessments over time, and the same issues and concerns being raised by review panels-even though the projects reviewed are operating in the same watershed and operated by the same proponent. Project environmental assessments must be managed, and coordinated, as part of a larger system of impact assessment, if project-by-project assessments are to provide a meaningful forum for learning and understanding cumulative change. The paper concludes with recommendations for improved project-based assessment practice in support of cumulative effects assessment and management.