Principles of Environmental Engineering and Science is well suited for a course in introductory environmental engineering for sophomore- or junior-level students. The emphasis is on engineering principles rather than on engineering design. The concept of mass balance is carried throughout the text as a tool for problem solving. The book includes more extensive coverage of chemistry, biology, and hydrology than other books in this field. The chemistry review in Chapter 2 and coverage of ethics will aid students in better understanding the engineering topics presented in the book.