Environmental science is an integration of material from the disciplines of biology, the physical sciences, and the social sciences. A prime objective of this book is to emphasize this interdependence and to maintain a balanced presentation of environmental issues important to the student. To this end, we have developed a principles oriented book that emphasizes concepts rather than the memorization of large amounts of raw data. This text is designed to explain how the underlying science, economics and politics affect important environmental issues. Adequate facts are given to illustrate and support the principles developed, and more importantly, facts and principles are integrated throughout the text.