This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book focuses on theories and techniques from a variety of disciplines that have a bearing on environmental assessment. It discusses the evolution of the main categories of environmental assessment methods: land-use planning tools; benefit-cost analysis, multiple-objective analysis, checklists, matrices, and networks; and modeling and simulation approaches. The book considers political theory to justify a participatory approach for bringing a broader set of values into an environmental assessment. It examines the value judgments implicit in an economic analysis and addresses the particular problems of placing monetary values on various aspects of environmental quality that are not exchanged in markets. It draws on theories from psychology which yield insights into how people experience and understand natural environments and how they form and express their value judgments.