The present report is intended to assist in the design and implementation of comparative risk assessment studies for the estimation of health and environmental impacts of various electricity generation options. It sets out the framework for carrying out such assessments and identifies the major technical issues and uncertainties in the assessment process. Contents: 1. Introduction; 2. Impacts and impact assessment: Basic concepts; 3. Methodological approach for estimating health and environmental impacts; 4. Calculation and use of impact indicators for comparative assessment; 5. Key methodological issues; Appendix I: Global climate change; Appendix II: Energy security.