Environmental decisions must satisfy a multitude of objectives and the matching of a plan, policy or project to such objectives is a matter of both facts and value judgements. This text provides a systematic approach to the structuring and measurement of value judgements, showing how they drive the decision process and how to make them transparent and effective in support of complex decisions. The value functions that the book describes provide a scheme for the exploration of human values and a tool for transforming them into an analytical model. A clear statement can then be made of the degree to which a decision has achieved its objectives, and how conflicting objectives may be addressed. This does not mean that there is no role for human judgement in the process. Complexity, often coupled with large information gaps, necessitates expert judgement, but the values adopted by the experts are themselves capable of being structured and measured according to the value function methodology presented here, even if the judgements themselves are qualitative and tentative. Value models for expert panels are also presented. The use of the methodology in practice is illustrated by a number of examples.