This book has a dual objective: first to introduce the reader to some of the most important and widespread environmental issues of the day; and second to illustrate the vital role played by mathematical models in investigating these issues. The subjects covered are ground water contamination, air pollution, and hazardous materials emergencies. These issues are presented in their full real-world context and are used to develop important classical mathematical themes. The emphasis throughout is on fundamental principles and concepts, not on achieving technical mastery of state-of-the-art models.