Focusing on the areas of engineering and infrastructure investment, this text begins with some general comments about the different decision components within the project planning process - the definition of objectives, the identification of alternative courses of action, the establishing of criteria, the evaluation of alternatives and the final recommendation. The authors highlight the ability of Multi-criteria Decision Aid to reconcile the economic, technical and environmental dimensions of the projects for its planners. They emphasize the complexity of this process, illustrating the importance of identifying the stakeholders within it, as they greatly influence the definition of the decision criteria. A brief case study illustrates these different aspects. Following a comparison of Cost Benefit Analysis and Multi-criteria Decision Aid, the introductory chapter sets out the structure of the book, with four subsequent chapters devoted to the methodology of ELECTRE and three outlining case studies involving different versions of ELECTRE. The chapters concentrating on the ELECTRE methodology first give an overview of the main MCDA methods before presenting the ELECTRE method in detail. Each chapter answers the following questions: (1) In what context should the ELECTRE methods be chosen? (2) Which version of the methods is most appropriate to apply to a given problem? Another chapter deals with a critical and delicate problem within MCDA - how to adequately assess the role played by each criterion in a given decision problem, and how this translates into an appropriate weighting for it. Each one covers a different civil engineering discipline and each uses a different version of ELECTRE. The final chapter on methodology presents some accessories which, when used with ELECTRE, can greatly enhance its usefulness in practice.