The third edition of this book was prepared to assist instructors in transport processes who are making partial use of SI units in their classes. Although some favor a sharp break with past systems of engineering units, we think there are good reasons why students should be educated using both the traditional engineering units and SI units. The quantities pounds, mass, and feet are embedded in our culture and will probably remain so for some time to come. It is sufficiently difficult for most students to learn the mathematical formulation of problems in engineering without adding to their burdens by expressing results entirely in unfamiliar units.