In economics, as in most other disciplines, we use models to investigate complex subjects such as relationships between the economy and the environment. Models are simplified characterizations of reality. For example, although a road map by design leaves out much detail, it is nonetheless a useful guide to reality. By showing how various locations relate to each other, a map gives an overall perspective. It cannot, however, capture all of the unique details that characterize any particular location. The map highlights only those characteristics that are crucial for the purpose at hand. The models in this text are similar. Through simplification, less detail is considered so that the main concepts and the relationships among them become clear.